[RISCV] isel (add (and X, 0x1FFFFFFFE), Y) as (SH1ADD (SRLI X, 1), Y)

This pattern is what we get after DAG combine for C code like this.

short *ptr1, *ptr2, *ptr3;
unsigned diff = ptr1 - ptr2;
return ptr3[diff];

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D126588