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

[X86] X86FixupVectorConstantsPass - attempt to replace full width fp vector constant loads with broadcasts on AVX+ 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.

NOTE: SSE3 targets can use MOVDDUP but not all SSE era CPUs can perform this as cheaply as a vector load, we will need to add scheduler model checks if we want to pursue this.

This is an updated commit of 98061013 after being reverted at a279a09a