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 a test.

The ConvertSelectToConcatVector is assuming it doesn't get vselects with
arguments of, for example, <undef, undef, true, true>. Those get taken
care of in the checks above its call.

Reviewers: nadav, delena, grosbach, hfinkel

Subscribers: llvm-commits

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209929 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/Generic/select.ll

@@ -192,4 +192,3 @@ define <1 x i32> @checkScalariseVSELECT(<1 x i32> %a, <1 x i32> %b) {
         %s = select <1 x i1> %cond, <1 x i32> %a, <1 x i32> %b
         ret <1 x i32> %s
 }
-