[SimplifyLibCalls] Add test to ensure transform is not executed if fast-math

attribute is not present.

During my refactor in r251595 I changed the behavior of optimizeSqrt(),
skipping the transformation if the function wasn't marked with unsafe-fp-math
attribute. This fixed a bug, as confirmed by Sanjay (before the optimization
was silently executed anyway), although it wasn't my primary aim.
This commit adds a test to ensure the code doesn't break again.

Reported by: Marcello Maggioni
Discussed with: Sanjay Patel

llvm-svn: 251747