[RISCV] Refactor SelectAddrRegImm to not depend on SelectBaseAddr.

SelectBaseAddr was a minor convenience to use since it already'
existed for vector load/store. D128187 is going to remove the other
uses of SelectBaseAddr so it has less reason to exist.

This patch removes the dependency on SelectBaseAddr and adds a new
SelectAddrFrameIndex to share some code with SelectFrameAddrRegImm.