gcc.dg/tree-ssa/ssa-dom-cse-2.c: xfail scan for mmix.

gcc.dg/tree-ssa/ssa-dom-cse-2.c: xfail scan for mmix.

Looking at the dump and the test, I guess all "64-bit targets" fail
the test for the reasons seen in the comment just above the dg-final,
whose last two lines make it to the patch context.  Maybe the xfail
target list can be shortened by removing most targets and use just
"lp64".  That doesn't cover it passing on x86_64 and s390x-zarch
though.

gcc/testsuite:
	* gcc.dg/tree-ssa/ssa-dom-cse-2.c: xfail scan for mmix.