[SystemZ] Use VGM whenever possible to load FP immediates.

isFPImmLegal() has been extended to recognize certain FP immediates that can
be built with VGM (Vector Generate Mask).

These scalar FP immediates (that were previously loaded from the constant
pool) are now selected as VGMF/VGMG in Select().

Review: Ulrich Weigand
https://reviews.llvm.org/D58003

llvm-svn: 353867