[x86] Minimize the parameters to this test for clarity.

The test has to do with DAG combines, and so it doesn't need the new
vector shuffle lowering to be effective. Also, it has a nice in-IR
triple string which we should really be using rather than command line
flags (unless it varies form RUN-line to RUN-line). Finally, I much
prefer letting LLVM synthesize the correct datalayout string from the
triple rather than baking one in here that will just become stale.