AMDGPU: Add option to disable CGP division expansion

The division expansions in AMDGPUCodeGenPrepare can't be relied on for
correctness, since they punt to later optimization and possibly
legalization in some cases. We still need a way to be able to write
tests for the legalizer versions of the expansion. This is mostly for
GlobalISel, since the expected optimzations is expecting aren't
implemented.

The interaction with the flag to expand 64-bit division in the IR is
pretty confusing, but these flags have different purposes.