Fix latent bug. This can't actually manifest at the moment, but is a time-bomb
for the next time someone adds something to this function.
Lower certain build_vectors to insertps instructions

Summary:
Vectors built with zeros and elements in the same order as another
(source) vector are optimized to be built using a single insertps
instruction.
Also optimize when we move one element in a vector to a different place
in that vector while zeroing out some of the other elements.

Further optimizations are possible, described in TODO comments.
I will be implementing at least some of them in the near future.

Added some tests for different cases where this optimization triggers.

Reviewers: nadav, delena, craig.topper

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3521