[LibCallSimplifier] use instruction-level fast-math-flags to transform pow(x,...

[LibCallSimplifier] use instruction-level fast-math-flags to transform pow(x, [small integer]) calls

This is a continuation of adding FMF to call instructions:
http://reviews.llvm.org/rL255555

As with D15937, the intent of the patch is to preserve the current behavior of the transform
except that we use the pow call's 'fast' attribute as a trigger rather than a function-level
attribute.

The TODO comment notes a potential follow-on patch that would propagate FMF to the new
instructions.

Differential Revision: http://reviews.llvm.org/D16122

llvm-svn: 258153