[llvm] [X86] X86FixupVectorConstants - shrink vector load to movsd/movsd/movd/movq 'zero upper' instructions (PR #79000)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@phoebewang This version always prefers the smallest possible constant load (and prefer vzload vs broadcast for the same constant width). This should also work when I add support for sextload/zextload handling for #71078

https://github.com/llvm/llvm-project/pull/79000