Move magic division optimization from morph to lowering

[mikedn]

Fixes #2183

This fixes all issues mentioned in 2183 by moving magic division to lowering and fixing GT_MULHI register requirements.

As mentioned in the issue doing magic division during global morph has a few problems - increased IR complexity, missed or partial CSE and LICM, missed magic division in cases that depend on constant propagation and folding.

It did, however, had one positive effect. In code like a = x / 3; b = x % 3; most of the common division code was CSEed. To preserve this useful side-effect I expanded x % 3 to x - (x / 3) * 3 in morph (there was already code to do this for ARM). This allows CSE to remove a redundant x / 3 (if any).

This expansion is currently done only if we know that lower will do magic division because magic division does the expansion anyway. We could do this for mod by power of 2 but the lowering or something else will need to fold (shl(shr(x, 2), 2) into a single and like the current mod by power of 2 does. But that's stuff for another PR.

[mmitche]

@dotnet-bot test Windows_NT x86 legacy_backend Checked Build and Test

[mikedn]

jit-diff summary:

Total bytes of diff: -722 (-0.01 % of base)
    diff is an improvement.
Total byte diff includes 0 bytes from reconciling methods
        Base had    0 unique methods,        0 unique bytes
        Diff had    0 unique methods,        0 unique bytes
Top file regressions by size (bytes):
          17 : System.Runtime.Numerics.dasm (0.03 % of base)
Top file improvements by size (bytes):
        -618 : System.Private.CoreLib.dasm (-0.02 % of base)
         -51 : System.Xml.ReaderWriter.dasm (-0.01 % of base)
         -19 : System.Reflection.Metadata.dasm (-0.01 % of base)
         -15 : System.Collections.Concurrent.dasm (-0.03 % of base)
          -9 : System.Runtime.InteropServices.dasm (-0.28 % of base)
11 total files with size differences.
Top method regessions by size (bytes):
          22 : System.Private.CoreLib.dasm - System.Threading.ManualResetEventSlim:Wait(int,struct):bool:this
          19 : System.Private.CoreLib.dasm - System.Globalization.Calendar:GetWeekOfYearOfMinSupportedDateTime(int,int):int:this
          19 : System.Runtime.Numerics.dasm - System.Numerics.Complex:GetHashCode():int:this
           9 : System.Private.CoreLib.dasm - System.Globalization.Calendar:GetWeekOfYearFullDays(struct,int,int):int:this
           4 : System.Private.CoreLib.dasm - System.Globalization.HijriCalendar:IsLeapYear(int,int):bool:this
Top method improvements by size (bytes):
         -96 : System.Private.CoreLib.dasm - System.Globalization.TimeSpanFormat:FormatCustomized(struct,ref,ref):ref
         -80 : System.Private.CoreLib.dasm - System.Globalization.TimeSpanFormat:FormatStandard(struct,bool,ref,int):ref
         -30 : System.Private.CoreLib.dasm - System.DateTimeFormat:FormatCustomized(struct,ref,ref,struct):ref
         -22 : System.Private.CoreLib.dasm - System.DateTimeFormat:AppendNumber(ref,long,int)
         -17 : System.Private.CoreLib.dasm - System.Globalization.JulianCalendar:AddMonths(struct,int):struct:this
121 total methods with size differences.

Elimination of useless mov instructions is responsible for most improvements. Improved elimination of redundant div code also helps here and there.

ManualResetEventSlim:Wait and Complex:GetHashCode regressed because they're now using magic division. GetWeekOfYearOfMinSupportedDateTime regressed due to not so fortunate register allocation (for example we end up using a larger lea instead of smaller add).

[jkotas]

cc @dotnet/jit-contrib

[russellhadley]

LGTM.