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.

llvm-svn: 225382