R600/SI: Remove VReg_32 register class

R600/SI: Remove VReg_32 register class

Use VGPR_32 register class instead.  These two register classes were
identical and having separate classes was causing
SIInstrInfo::isLegalOperands() to be overly conservative in some cases.

This change is necessary to prevent future paches from missing a folding
opportunity in fneg-fabs.ll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225382 91177308-0d34-0410-b5e6-96231b3b80d8