[RISCV] Refactor the common combines for SELECT_CC and BR_CC into a helper function.

[RISCV] Refactor the common combines for SELECT_CC and BR_CC into a helper function.

The only difference between the combines were the calls to getNode
that include the true/false values for SELECT_CC or the chain
and branch target for BR_CC.

Wrap the rest of the code into a helper that reads LHS, RHS, and
CC and outputs new values and a bool if a new node needs to be
created.