[RISCV] Constrain register class before replaceRegWith in RISCVMergeBaseOffset.

[craig.topper]

Summary

The register being replaced might have a more restrictive register
class due to requirements of the using instruction. We should
constrain the register class to preserve any restrictions.

This was found in our downstream on a custom instruction. I don't
have a test case for upstream currently.

[reames]

LGTM

llvm/lib/Target/RISCV/RISCVMergeBaseOffset.cpp

 void RISCVMergeBaseOffsetOpt::foldOffset(MachineInstr &Hi, MachineInstr &Lo,
                                          MachineInstr &Tail, int64_t Offset) {
   assert(isInt<32>(Offset) && "Unexpected offset");
   // Put the offset back in Hi and the Lo
   Hi.getOperand(1).setOffset(Offset);
   if (Hi.getOpcode() != RISCV::AUIPC)
     Lo.getOperand(2).setOffset(Offset);
   // Delete the tail instruction.
+  MRI->constrainRegClass(Lo.getOperand(0).getReg(),
+                         MRI->getRegClass(Tail.getOperand(0).getReg()));
   MRI->replaceRegWith(Tail.getOperand(0).getReg(), Lo.getOperand(0).getReg());
   Tail.eraseFromParent();
   LLVM_DEBUG(dbgs() << "  Merged offset " << Offset << " into base.\n"
                     << "     " << Hi << "     " << Lo;);
 }
 
 // Detect patterns for large offsets that are passed into an ADD instruction.
 // If the pattern is found, updates the offset in Hi and Lo instructions