[x86] Teach the new vector shuffle lowering to aggressively form MOVSS
and MOVSD nodes for single element vector inserts.

This is particularly important because a number of patterns in the
backend detect these patterns and leverage them to simplify things. It
also fixes quite a few of the insertion bad code examples. However, it
regresses a specific area: when available, blendps and blendpd are
*dramatically* faster than movss and movsd respectively. But it doesn't
really work to form the blend logic first because the blends *aren't* as
crazy efficient when the data is coming from memory anyways, and thus
will have a movss or movsd regardless. Also, doing that would block
a bunch of the patterns that this is designed to hit.

So my plan is to go into the patterns for lowering MOVSS and MOVSD and
lower them via blends when available. However that's a pretty invasive
restructuring so it will need to be a follow-up patch.

I have already gone into the patterns to lower MOVSS and MOVSD from
memory using MOVLPD, etc. Without that, several of the test cases
I already have regress.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218985 91177308-0d34-0410-b5e6-96231b3b80d8
Adding skeleton for unit testing Hexagon Code Emission

Adding and modifying CMakeLists.txt files to run unit tests under
unittests/Target/* if the directory exists.  Adding basic unit test to check
that code emitter object can be retrieved.

Differential Revision: http://reviews.llvm.org/D5523
Change by: Colin LeMahieu

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218986 91177308-0d34-0410-b5e6-96231b3b80d8