[MachineCombiner][RISCV] Make hasReassociableSibling virtual and override it for RISCV

To check reassociation correctness for RISCV, we must ensure that the root and
it's sibling have equal rounding modes (for floating point instructions).
`hasReassociableSibling` is a good place to make additional target-dependend
checks.

This patch allows us to enable default machine combiner mechanism to gather
reassociation candidates on RISCV.

Differential Revision: https://reviews.llvm.org/D138302