[X86] Prefer fpext(splat(X)) to splat(fpext(x)).

This patch is to fix regression of D122875. X86 has fpext instructions
supporting rmb form, which takes advantage of fpext(fplat(X)) than
splat(fpext(X)).

Reviewed By: RKSimon, skan

Differential Revision: https://reviews.llvm.org/D141657