[x86] Teach the X86 backend to DAG-combine SSE2 shuffles that are
trivially redundant.

This fixes several cases in the new vector shuffle lowering algorithm
which would generate redundant shuffle instructions for the sake of
simplicity.

I'm also deleting a testcase which was somewhat ridiculous. It was
checking for a bug in 2007 about incorrectly transforming shuffles by
looking for the string "-86" in the output of a pretty substantial
function. This test case doesn't seem to have any value at this point.

Differential Revision: http://reviews.llvm.org/D4240