Cost model support for lowered math builtins.

We make the cost for calling libm functions extremely high as emitting the
calls is expensive and causes spills (on x86) so performance suffers. We still
vectorize important calls like ceilf and friends on SSE4.1. and fabs.

Differential Revision: http://llvm-reviews.chandlerc.com/D466

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176287 91177308-0d34-0410-b5e6-96231b3b80d8
Tidy up; no functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176288 91177308-0d34-0410-b5e6-96231b3b80d8
Don't add an attribute that already exists and don't remove an attribute that doesn't exist.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176289 91177308-0d34-0410-b5e6-96231b3b80d8
Brag about function call vectorization in the docs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176292 91177308-0d34-0410-b5e6-96231b3b80d8