[x86] Teach the x86 DAG combiner to form MOVSLDUP and MOVSHDUP

instructions when it finds an appropriate pattern.

These are lovely instructions, and its a shame to not use them. =] They
are fast, and can hand loads folded into their operands, etc.

I've also plumbed the comment shuffle decoding through the various
layers so that the test cases are printed nicely.

llvm-svn: 217758