[InstCombine] x /c fabs(x) -> copysign(1.0, x)

Summary:
x / fabs(x) -> copysign(1.0, x)
fabs(x) / x -> copysign(1.0, x)

Reviewers: spatel, foad, RKSimon, efriedma

Reviewed By: spatel

Subscribers: lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65898

llvm-svn: 368570