JIT: Don't allow broadcast lowering to remove cast from decomposed long

[saucecontrol]

Fixes #115202

I was unable to create a minimal repro for this, but using the latest SPMI context from #115202 (comment), I found the cause.
 
The problematic IR starts out as:

N025 (???,???) [016563] -ACXG+-----                t16563 = *  CALL      long   <unknown method>
                                                            /--*  t16563 long   
N026 (???,???) [016564] -ACXG+-----                t16564 = *  CAST      int <- long
                                                            /--*  t16564 int    
N027 (???,???) [016565] -ACXG+-----                t16565 = *  HWINTRINSIC simd32 32 int Create

This Vector256.Create is lowered to Avx2.BroadcastScalarToVector256 <- Vector256.CreateScalarUnsafe <- CAST(int <- long).

Then the containment check for broadcast, which looks for that specific pattern, is eating both the CreateScalarUnsafe and the GT_CAST.

Resulting in an uncontained GT_LONG:

               [038143] -----------                t38143 =    LCL_FLD   int    V1485 tmp1445    [+0]
               [038144] -----------                t38144 =    LCL_FLD   int    V1485 tmp1445    [+4]
                                                            /--*  t38143 int    
                                                            +--*  t38144 int    
               [038145] -----------                t38145 = *  LONG      long  
                                                            /--*  t38145 long   
N027 (???,???) [016565] -ACXG+-----                t16565 = *  HWINTRINSIC simd32 32 int BroadcastScalarToVector256

In this case, the cast should have been preserved, and it should contain the GT_LONG.

Correct IR after lowering:

               [038143] -----------                t38143 =    LCL_FLD   int    V1485 tmp1445    [+0]
               [038144] -----------                t38144 =    LCL_FLD   int    V1485 tmp1445    [+4]
                                                            /--*  t38143 int    
                                                            +--*  t38144 int    
               [038145] -c---------                t38145 = *  LONG      long  
                                                            /--*  t38145 long   
N026 (???,???) [016564] -ACXG+-----                t16564 = *  CAST      int <- long
                                                            /--*  t16564 int    
N027 (???,???) [016565] -ACXG+-----                t16565 = *  HWINTRINSIC simd32 32 int BroadcastScalarToVector256

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[tannergooding]

CC. @dotnet/jit-contrib, @jakobbotsch for secondary review.

It would be nice if we could get a small regression test added for this, but much like @saucecontrol I wasn't able to get a minimal repro either. Even the smaller sample given in the issue I was never able to get to reproduce locally, although it repro's in the SPMI context.

[jakobbotsch]

Thanks for fixing this @saucecontrol. I am ok with not adding a test if it is unreasonably hard to come up with one. Fuzzlyn has also had issues reducing these test cases. Although it would be interesting to understand why that is the case.