[RISCV] Use analyzeBranch in RISCVRedundantCopyElimination.

The existing code was incorrect if we had more than one conditional
branch instruction in a basic block. Though I don't think that will
occur, using analyzeBranch detects that as an unsupported case.

Overall this results in simpler code in RISCVRedundantCopyElimination.

Reviewed By: reames, kito-cheng

Differential Revision: https://reviews.llvm.org/D132347