[Inliner] Merge the attributes of the caller and callee functions

This patch turns off the fast-math optimization attribute on the caller
if the callee's fast-math attribute is not turned on.

For example,

- before inlining
 caller: "less-precise-fpmad"="true"
 callee: "less-precise-fpmad"="false"

- after inlining
 caller: "less-precise-fpmad"="false"

Alternatively, it's possible to block inlining if the caller's and
callee's attributes don't match. If this approach is preferable to the
one in this patch, we can discuss post-commit.

rdar://problem/19836465

Differential Revision: http://reviews.llvm.org/D7802

llvm-svn: 257575