[RISCV] Constrain register class before replaceRegWith in RISCVMergeBaseOffset.

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.

Differential Revision: https://reviews.llvm.org/D154920