gcc.dg/loop-8.c: Skip for mmix.

gcc.dg/loop-8.c: Skip for mmix.

This test fails for mmix for (almost) the same reason it would fail
for e.g. mipsel-elf: the end-condition of the loop tests against a
register set to a constant, and that register is (one of) the
"unexpected IV" moved out of the loop "without introducing a new
temporary register" and making the dump contain more than one
"Decided", causing a non-matching loop2 dump.  The test should
probably have been restricted to just the original target for which a
problem was observed to be fixed.

gcc/testsuite:
	* gcc.dg/loop-8.c: Skip for mmix.