[SimplifyLibCalls] Tranform log(pow(x, y)) -> y*log(x).

This one is enabled only under -ffast-math. There are cases where the
difference between the value computed and the correct value is huge
even for ffast-math, e.g. as Steven pointed out:

x = -1, y = -4
log(pow(-1), 4) = 0
4*log(-1) = NaN

I checked what GCC does and apparently they do the same optimization
(which result in the dramatic difference). Future work might try to
make this (slightly) less worse.

Differential Revision:	http://reviews.llvm.org/D14400

llvm-svn: 254263