AMDGPU: Add flag to disable fdiv processing in IR pass

We kind of have to have multiple implementations of fdiv split between
the two selectors with some pre-processing. Add yet another test to
check for consistency of interpretation of flag combinations. We have
quite a bit of test redundancy here already, but there are so many
possible interesting permutations it's unwieldy to cover every detail
in any one of them. We have a number of overlapping fdiv tests but
it's hard to follow everything going on as it is.