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

[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 ab4b9248 after being reverted at 78de45fd