Only have codegen turn fdiv by a constant into fmul by the reciprocal

Only have codegen turn fdiv by a constant into fmul by the reciprocal
when -ffast-math, i.e. don't just always do it if the reciprocal can
be formed exactly.  There is already an IR level transform that does
that, and it does it more carefully.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154296 91177308-0d34-0410-b5e6-96231b3b80d8