Pre-commit for D62807 - which adds DAG [us]itofp(undef) --> 0 constant fold

llvm-svn: 362396

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

Subregister liveness applies selectively to register classes with certain
properties. Make sure that when it's enabled, it applies to a given virtual
register (in virtual register rewriter).

llvm-svn: 339784