[RISCV] Add more SHXADD isel patterns.

This handles the code we get for

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

The shift right and the shl will get DAG combined into
(shl (and X, 0xfffffffe), 1). We have custom isel to match the
shl+and, but with Zba the (add (shl X, 1), Y) part will get
matched and leave the and to be iseled by itself. This commit
adds a larger pattern that includes the and.