[AMDGPU] Simplify 64-bit division/remainder expansion

The old expansion open-coded a 64-bit addition in a strange way, by
adding the high parts *without* carry-in from the low part, and then
adding the carry back in later on. Fixing this saves a couple of
instructions and makes the code much easier to understand.

Differential Revision: https://reviews.llvm.org/D113679