[RISCV] isel (add (and X, 0xFFFFFFFE), Y) as (SH1ADD (SRLIW X, 1), Y).

Similar for SH2ADD and SH3ADD.

This is what we get from

int foo(int* x, unsigned y) {
  return x[y >> 1];
}

This allows us to avoid materializing 0xFFFFFFFE into a register.