Assertion prop does not properly truncate stores to small-typed args

[jakobbotsch]

public static void Main()
{
    System.Console.WriteLine(M(0));
}

[MethodImpl(MethodImplOptions.NoInlining)]
public static int M(byte arg0)
{
    IL.Emit.Ldc_I4(-1);
    IL.Emit.Starg(0);
    IL.Emit.Ldarg(0);
    return IL.Return<int>();
}

Expected: 255
Actual: 255 in debug, -1 in release

...

*************** In fgMorphBlocks()

Morphing BB01 of 'Program:M(ubyte):int'

fgMorphTree BB01, STMT00000 (before)
               [000002] -A---------                         ▌  ASG       ubyte 
               [000001] D------N---                         ├──▌  LCL_VAR   ubyte  V00 arg0         
               [000000] -----------                         └──▌  CNS_INT   int    -1
Notify VM instruction set (SSE2) must be supported.
GenTreeNode creates assertion:
               [000002] -A---------                         ▌  ASG       ubyte 
In BB01 New Local Constant Assertion: V00 == -1, index = #01

fgMorphTree BB01, STMT00001 (before)
               [000004] -----------                         ▌  RETURN    int   
               [000003] -----------                         └──▌  LCL_VAR   ubyte  V00 arg0         

Assertion prop in BB01:
Constant Assertion: V00 == -1, index = #01
               [000003] -----------                         ▌  CNS_INT   int    -1

fgMorphTree BB01, STMT00001 (after)
               [000004] -----+-----                         ▌  RETURN    int   
               [000003] -----+-----                         └──▌  CNS_INT   int    -1

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Issue Details

---

public static void Main()
{
    System.Console.WriteLine(M(0));
}

[MethodImpl(MethodImplOptions.NoInlining)]
public static int M(byte arg0)
{
    IL.Emit.Ldc_I4(-1);
    IL.Emit.Starg(0);
    IL.Emit.Ldarg(0);
    return IL.Return<int>();
}

Expected: 255
Actual: 255 in debug, -1 in release

...

*************** In fgMorphBlocks()

Morphing BB01 of 'Program:M(ubyte):int'

fgMorphTree BB01, STMT00000 (before)
               [000002] -A---------                         ▌  ASG       ubyte 
               [000001] D------N---                         ├──▌  LCL_VAR   ubyte  V00 arg0         
               [000000] -----------                         └──▌  CNS_INT   int    -1
Notify VM instruction set (SSE2) must be supported.
GenTreeNode creates assertion:
               [000002] -A---------                         ▌  ASG       ubyte 
In BB01 New Local Constant Assertion: V00 == -1, index = #01

fgMorphTree BB01, STMT00001 (before)
               [000004] -----------                         ▌  RETURN    int   
               [000003] -----------                         └──▌  LCL_VAR   ubyte  V00 arg0         

Assertion prop in BB01:
Constant Assertion: V00 == -1, index = #01
               [000003] -----------                         ▌  CNS_INT   int    -1

fgMorphTree BB01, STMT00001 (after)
               [000004] -----+-----                         ▌  RETURN    int   
               [000003] -----+-----                         └──▌  CNS_INT   int    -1

Author:	jakobbotsch
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[jakobbotsch]

This is a small repro of a problem in #70518 (comment).