Scale cloned loop block weights

[BruceForstall]

The slow path of cloned loops are scaled to keep only 1% of the
original block weights, while the original loop keeps 99% of the weight.
It could be argued that the cloned loop slow path, as currently implemented,
should only be executed during exceptional paths, and thus should be
marked "run rarely", but don't do that for now.

There are many diffs, due to weights changing. Mostly, fewer CSEs in cloned
loop slow paths. E.g., aspnet spmi diffs included below.

Also, add a header comment for optRedirectBlock (previous change follow-up).
In addition, I didn't like the semantics of the addPreds flag -- it didn't
match well with the rest of the function -- so I removed it and added the appropriate
changes in optCloneLoop (the only caller that used that) to compensate.

I added code to optEnsureUniqueHead, only used by loop cloning, to add and update
appropriate preds edges. It turns out, however, this function is currently dead
code, due to other limitations in loop cloning. I'll leave the changes anyway, in
case those limitations are removed later. This part of the change I verified was no-diff
(and verified the code wasn't even executed in our SPMI collections).

Summary of Code Size diffs:
(Lower is better)

Total bytes of base: 18811
Total bytes of diff: 18749
Total bytes of delta: -62 (-0.33% of base)
    diff is an improvement.

Detail diffs

Top file regressions (bytes):
          23 : 19019.dasm (9.91% of base)
          23 : 38588.dasm (9.91% of base)
           8 : 39269.dasm (1.15% of base)
           3 : 19897.dasm (0.94% of base)
           2 : 40501.dasm (1.43% of base)

Top file improvements (bytes):
         -37 : 13173.dasm (-2.41% of base)
         -37 : 14302.dasm (-2.41% of base)
         -17 : 31284.dasm (-0.84% of base)
         -17 : 34351.dasm (-0.83% of base)
          -5 : 31157.dasm (-0.25% of base)
          -5 : 34245.dasm (-0.25% of base)
          -3 : 38167.dasm (-0.49% of base)

12 total files with Code Size differences (7 improved, 5 regressed), 4 unchanged.

Top method regressions (bytes):
          23 ( 9.91% of base) : 19019.dasm - ObjectEqualityComparer`1:IndexOf(ref,__Canon,int,int):int:this
          23 ( 9.91% of base) : 38588.dasm - ObjectEqualityComparer`1:IndexOf(ref,__Canon,int,int):int:this
           8 ( 1.15% of base) : 39269.dasm - RuntimeMethodInfo:MakeGenericMethod(ref):MethodInfo:this
           3 ( 0.94% of base) : 19897.dasm - ControllerActionInvoker:PrepareArguments(IDictionary`2,ObjectMethodExecutor):ref
           2 ( 1.43% of base) : 40501.dasm - SslCredKey:Equals(SslCredKey):bool:this

Top method improvements (bytes):
         -37 (-2.41% of base) : 13173.dasm - DefaultTypeMap:FindConstructor(ref,ref):ConstructorInfo:this
         -37 (-2.41% of base) : 14302.dasm - DefaultTypeMap:FindConstructor(ref,ref):ConstructorInfo:this
         -17 (-0.84% of base) : 31284.dasm - EntityMaterializerInjectingExpressionVisitor:MaterializeEntity(EntityShaperExpression,ParameterExpression,ParameterExpression,ParameterExpression,ParameterExpression):Expression:this
         -17 (-0.83% of base) : 34351.dasm - EntityMaterializerInjectingExpressionVisitor:MaterializeEntity(EntityShaperExpression,ParameterExpression,ParameterExpression,ParameterExpression,ParameterExpression):Expression:this
          -5 (-0.25% of base) : 31157.dasm - EntityShaperExpression:GenerateMaterializationCondition(IEntityType,bool):LambdaExpression:this
          -5 (-0.25% of base) : 34245.dasm - EntityShaperExpression:GenerateMaterializationCondition(IEntityType,bool):LambdaExpression:this
          -3 (-0.49% of base) : 38167.dasm - RuntimeMethodInfo:Equals(Object):bool:this

Top method regressions (percentages):
          23 ( 9.91% of base) : 19019.dasm - ObjectEqualityComparer`1:IndexOf(ref,__Canon,int,int):int:this
          23 ( 9.91% of base) : 38588.dasm - ObjectEqualityComparer`1:IndexOf(ref,__Canon,int,int):int:this
           2 ( 1.43% of base) : 40501.dasm - SslCredKey:Equals(SslCredKey):bool:this
           8 ( 1.15% of base) : 39269.dasm - RuntimeMethodInfo:MakeGenericMethod(ref):MethodInfo:this
           3 ( 0.94% of base) : 19897.dasm - ControllerActionInvoker:PrepareArguments(IDictionary`2,ObjectMethodExecutor):ref

Top method improvements (percentages):
         -37 (-2.41% of base) : 13173.dasm - DefaultTypeMap:FindConstructor(ref,ref):ConstructorInfo:this
         -37 (-2.41% of base) : 14302.dasm - DefaultTypeMap:FindConstructor(ref,ref):ConstructorInfo:this
         -17 (-0.84% of base) : 31284.dasm - EntityMaterializerInjectingExpressionVisitor:MaterializeEntity(EntityShaperExpression,ParameterExpression,ParameterExpression,ParameterExpression,ParameterExpression):Expression:this
         -17 (-0.83% of base) : 34351.dasm - EntityMaterializerInjectingExpressionVisitor:MaterializeEntity(EntityShaperExpression,ParameterExpression,ParameterExpression,ParameterExpression,ParameterExpression):Expression:this
          -3 (-0.49% of base) : 38167.dasm - RuntimeMethodInfo:Equals(Object):bool:this
          -5 (-0.25% of base) : 31157.dasm - EntityShaperExpression:GenerateMaterializationCondition(IEntityType,bool):LambdaExpression:this
          -5 (-0.25% of base) : 34245.dasm - EntityShaperExpression:GenerateMaterializationCondition(IEntityType,bool):LambdaExpression:this

12 total methods with Code Size differences (7 improved, 5 regressed), 4 unchanged.

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[BruceForstall]

@AndyAyersMS @dotnet/jit-contrib PTAL

[AndyAyersMS]

LGTM