[X86] X86FixupVectorConstants - shrink vector load to movsd/movsd/movd/movq...

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

If we're loading a vector constant that is known to be zero in the upper elements, then attempt to shrink the constant and just scalar load the lower 32/64 bits.

Always chose the vzload/broadcast with the smallest constant load, and prefer vzload over broadcasts for same bitwidth to avoid domain flips (mainly a AVX1 issue).

Fixes #73783