R600/SI: Make SIInstrInfo::isOperandLegal() more strict

A register operand that has a common sub-class with its instruction's
defined register class is not always legal.  For example,
SReg_32 and M0Reg both have a common sub-class, but we can't
use an SReg_32 in instructions that expect a M0Reg.

This prevents the llvm.SI.sendmsg.ll test from failing when the fold
operand pass is added.