[AMDGPU] Fix and simplify AMDGPUCodeGenPrepare::expandDivRem32

Fix the division/remainder algorithm by adding a second quotient
refinement step, which is required in some cases like
0xFFFFFFFFu / 0x11111111u (https://bugs.llvm.org/show_bug.cgi?id=46212).

Also document, rewrite and simplify it by ensuring that we always have a
lower bound on inv(y), which simplifies the UNR step and the quotient
refinement steps.

Differential Revision: https://reviews.llvm.org/D83381