[PATCH] Convert a vselect into a concat_vector if possible

[Filipe Cabecinhas]

If half is undef, and the other half is X (either true or false), this will
fall under the if statements before the call to this function in
visitSELECT. Around line 4707 (isBuildVectorAll{Zeros,Ones}). I also added
a comment at the call site of this function.

Filipe

http://reviews.llvm.org/D3916