AMDGPU: Add option to expand 64-bit integer division in IR

I didn't realize we were already expanding 24/32-bit division here
already. Use the available IntegerDivision utilities. This uses loops,
so produces significantly smaller code than the inline DAG expansion.

This now requires width reductions of 64-bit divisions before
introducing the expanded loops.

This helps work around missing legalization in GlobalISel for
division, which are the only remaining core instructions that didn't
work at all.

I think this is plausibly a better implementation than exists in the
DAG, although turning it on by default misses out on the constant
value optimizations and also needs benchmarking.