[X86] X86FixupVectorConstantsPass - attempt to replace full width integer vector...

[X86] X86FixupVectorConstantsPass - attempt to replace full width integer vector constant loads with broadcasts on AVX2+ targets (REAPPLIED)

lowerBuildVectorAsBroadcast will not broadcast splat constants in all cases, resulting in a lot of situations where a full width vector load that has failed to fold but is loading splat constant values could use a broadcast load instruction just as cheaply, and save constant pool space.

This is an updated commit of ab4b924832ce26c21b88d7f82fcf4992ea8906bb after being reverted at 78de45fd4a902066617fcc9bb88efee11f743bc6