Convert a vselect into a concat_vector if possible

Summary:
If both vector args to vselect are concat_vectors and the condition is
constant and picks half a vector from each argument, convert the vselect
into a concat_vectors.

Added tests.

Reviewers: nadav, delena, grosbach

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3916