[RISCV] Enable machine copy propagation for copy-like instructions

Like what has been done in AArch64 (D125335).

We enable this under `-O2` to show the codegen diffs here but we
may only do this under `-O3` like AArch64.

There are two cases that we may produce these eliminable copies:
1. ISel of `FrameIndex`. Like `rvv/fixed-vectors-calling-conv.ll`.
2. Tail duplication. Like `select-optimize-multiple.ll`.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144535