[X86] X86FixupVectorConstantsPass - add support for VPMOVSX/VPMOVZX constant extension

[RKSimon]

X86FixupVectorConstantsPass currently just handles folding full vector loads to broadcasts, but we're missing the opportunity to use sign/zero extension loads for non-uniform cases.

void foo(const int *src, float *dst) {
    for (int i = 0; i != 16; ++i) {
        *dst++ = (float)(*src++ + ((i % 8) + 1));
    }
}

llc -mcpu=x86-64-v3

foo(int const*, float*): # @foo(int const*, float*)
  vmovdqa .LCPI0_0(%rip), %ymm0 # ymm0 = [1,2,3,4,5,6,7,8]
  vpaddd (%rdi), %ymm0, %ymm1
  vcvtdq2ps %ymm1, %ymm1
  vmovups %ymm1, (%rsi)
  vpaddd 32(%rdi), %ymm0, %ymm0
  vcvtdq2ps %ymm0, %ymm0
  vmovups %ymm0, 32(%rsi)
  vzeroupper
  retq

We can reduce the size of the constant pool entry by replacing the vmovdqa load with vpmovzxbd

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

X86FixupVectorConstantsPass currently just handles folding full vector loads to broadcasts, but we're missing the opportunity to use sign/zero extension loads for non-uniform cases. ```c void foo(const int *src, float *dst) { for (int i = 0; i != 16; ++i) { *dst++ = (float)(*src++ + ((i % 8) + 1)); } } ``` llc -mcpu=x86-64-v3 ```asm foo(int const*, float*): # @foo(int const*, float*) vmovdqa .LCPI0_0(%rip), %ymm0 # ymm0 = [1,2,3,4,5,6,7,8] vpaddd (%rdi), %ymm0, %ymm1 vcvtdq2ps %ymm1, %ymm1 vmovups %ymm1, (%rsi) vpaddd 32(%rdi), %ymm0, %ymm0 vcvtdq2ps %ymm0, %ymm0 vmovups %ymm0, 32(%rsi) vzeroupper retq ``` We can reduce the size of the constant pool entry by replacing the vmovdqa load with vpmovzxbd