gcc.dg/tree-ssa/ssa-dse-26.c: Skip on mmix-knuth-mmixware

gcc.dg/tree-ssa/ssa-dse-26.c: Skip on mmix-knuth-mmixware

Commit r12-432, rewriting the dg-stuff, reverted the
adjustment for mmix-knuth-mmixware that I added in r11-2335.
(See those commits for context.)

Hopefully this variant will age better, just skipping it
with a trivial extra line less prone to pile-on.  (Not much
is won by covering this generic case for MMIX too; might as
well skip it.)

Beware that the dg-skip-if text can't say
"temporary variables are not x and y but x::3 and y::4"
because that leads to (on one line):

ERROR: gcc.dg/tree-ssa/ssa-dse-26.c: can't set "{temporary
 variables are not x and y but x::3 and y::4} {
 mmix-knuth-mmixware }": parent namespace doesn't exist for
 " dg-skip-if 4 "temporary variables are not x and y but
 x::3 and y::4" { mmix-knuth-mmixware } "

gcc/testsuite:
	* gcc.dg/tree-ssa/ssa-dse-26.c: Skip on mmix-knuth-mmixware.