[X86] Make the code that creates fmaddsub from build_vector of extracts and...

[X86] Make the code that creates fmaddsub from build_vector of extracts and inserts functional and add tests.

Summary:
We had no tests for this and we couldn't do the optimization because of a bad use count check. We need to know how many non-undef pieces of the build vector were filled in and ensure our use count is equal to that. But on the shuffle combine version we need the use count to be 2.

The missing coverage was noticed during the review of D40335.

Reviewers: RKSimon, zvi, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41133

llvm-svn: 320950