[AMDGPU] Early expansion of 32 bit udiv/urem

This allows hoisting of a common code, for instance if denominator
is loop invariant. Current change is expansion only, adding licm to
the target pass list going to be a separate patch. Given this patch
changes to codegen are minor as the expansion is similar to that on
DAG. DAG expansion still must remain for R600.

Differential Revision: https://reviews.llvm.org/D48586

llvm-svn: 335868