Remove redundant sign/zero extension for SIMD broadcasts

[EgorBo]

Closes #108820

static Vector512<sbyte> Create1(ulong value) =>
    Vector512.Create((sbyte)value);

; Method Bencha:Create1(ulong)
-      movsx    rdx, dl
       vpbroadcastb zmm0, edx
       vmovups  zmmword ptr [rcx], zmm0
       mov      rax, rcx
       vzeroupper 
       ret      
-; Total bytes of code: 23
+; Total bytes of code: 19

Diffs

[EgorBo]

@tannergooding @kunalspathak @dotnet/jit-contrib PTAL

[xtqqczze]

Diffs show additional improvement with memory source:

-       mov      eax, dword ptr [rbp-0x4C]
-       movzx    rax, ax
-       vpbroadcastw ymm0, eax
+       vpbroadcastw ymm0, word  ptr [rbp-0x4C]

[xtqqczze]

@EgorBo

Missed redundant sign-extension with memory source:

System.PackedSpanHelpers:IndexOf[System.SpanHelpers+DontNegate`1[short],System.PackedSpanHelpers+NopTransform](byref,short,int):int (Instrumented Tier0)

        mov      eax, dword ptr [rbp-0x34]
        cwde     
        vpbroadcastb xmm0, eax

[tannergooding]

I'm confident that changes like this are better:

Diffs show additional improvement with memory source:

-       mov      eax, dword ptr [rbp-0x4C]
-       movzx    rax, ax
-       vpbroadcastw ymm0, eax
+       vpbroadcastw ymm0, word  ptr [rbp-0x4C]

I'm not so confident on changes like:

-      movsx    rdx, dl
       vpbroadcastb zmm0, edx

movzx and movsx can avoid stalls otherwise caused by a partial write/merge and so while removing them should be better in the case that a previous instruction initialized either the full 32-bit or 64-bit state, it will likely be worse in cases where something instead only wrote the lower 8/16-bits of state prior to the broadcast.

It might be that the typical JIT patterns are already avoiding such 8/16-bit register writes in most cases, but it would be good to double check this.

[tannergooding]

Should this rather check for varTypeIsInteger on both?

We have several types (integer, floating, simd, and mask) so a negative test may accidentally include the wrong things

[EgorBo]

movzx and movsx can avoid stalls otherwise caused by a partial write/merge and so while removing them should be better in the case that a previous instruction initialized either the full 32-bit or 64-bit state, it will likely be worse in cases where something instead only wrote the lower 8/16-bits of state prior to the broadcast.

It might be that the typical JIT patterns are already avoiding such 8/16-bit register writes in most cases, but it would be good to double check this.

I can probably bench it (my bot is currently down), but looks like native compilers don't emit any movs e.g. https://godbolt.org/z/hKrGnz7nj