[LibCallSimplifier] use instruction-level fast-math-flags to transform log calls
Also, add tests to verify that we're checking 'fast' on both calls of each transform pair,
tighten the CHECK lines, and give the tests more meaningful names.
This is a continuation of:
http://reviews.llvm.org/rL255555
http://reviews.llvm.org/rL256871
http://reviews.llvm.org/rL256964
http://reviews.llvm.org/rL257400
http://reviews.llvm.org/rL257404
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257414
91177308-0d34-0410-b5e6-
96231b3b80d8