[InstCombine] remove compound fdiv pattern folds

These are fdiv-with-constant-divisor, so they already become
reciprocal multiplies. The last gap for vector ops should be
closed with rL325590.

It's possible that we're missing folds for some edge cases 
with denormal intermediate constants after deleting these,
but there are no tests for those patterns, and it would be 
better to handle denormals more consistently (and less 
conservatively) as noted in TODO comments.

llvm-svn: 325595