[RISCV] Add isel patterns to select slli+shXadd.uw.

This matches what we get for something like.
%0 = shl i32 %x, C
%1 = zext i32 %0 to i64
%2 = getelementptr i32, ptr %y, %1

The shift before the zext and the shift implied by the GEP get
combined with an AND after them. We need to split it back into
2 shifts so we can fold one into shXadd.uw.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D137886