R600/SI: Simplify and fix handling of VOP2 in SIInstrInfo::legalizeOperands

We were incorrectly assuming that all VOP2 instructions can read SGPRs
in Src0, but this is not true for instructions that read carry-in from
VCC.

The old logic has been replaced with new logic which checks the defined
register classes of the VOP2 instruction to determine whether or not to
legalize the operands.

llvm-svn: 214465