[X86] Fixes bug in build_vector v4x32 lowering

[X86] Fixes bug in build_vector v4x32 lowering

r222375 made some improvements to build_vector lowering of v4x32 and v4xf32 into an insertps, but it missed a case where:

1. A single extracted element is used twice.
2. The lower of the two non-zero indexes should be preserved, and the higher should be used for the dest mask.

This caused a crash, since the source value for the insertps ends-up uninitialized.

Differential Revision: http://reviews.llvm.org/D6377

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222635 91177308-0d34-0410-b5e6-96231b3b80d8