[AMDGPU] Set fast-math flags on functions given the options

We have a single library build without relaxation options.
When inlined library functions remove fast math attributes
from the functions they are integrated into.

This patch sets relaxation attributes on the functions after
linking provided corresponding relaxation options are given.
Math instructions inside the inlined functions remain to have
no fast flags, but inlining does not prevent fast math
transformations of a surrounding caller code anymore.

Differential Revision: https://reviews.llvm.org/D38325

llvm-svn: 314568