[LibCallSimplifier] use instruction-level fast-math-flags to transform sqrt calls

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

The intent of the patch is to preserve the current behavior of the transform except
that we use the sqrt instruction's 'fast' attribute as a trigger rather than the
function-level attribute.

But this raises a bug noted by the new FIXME comment.

In order to do this transform:
sqrt((x * x) * y) ---> fabs(x) * sqrt(y)

...we need all of the sqrt, the first fmul, and the second fmul to be 'fast'. 
If any of those ops is strict, we should bail out.

Differential Revision: http://reviews.llvm.org/D15937

llvm-svn: 257400