gcc.dg/tree-ssa/reassoc-20.c: Adjust for mmix.

gcc.dg/tree-ssa/reassoc-20.c: Adjust for mmix.

The expression of interest looks like "e_5 = a::0_1 + b::1_2;" for
mmix-knuth-mmixware, while other targets have a "." instead of the
"::".  ISTM the most useful change here is not to disable the test,
but to add an optional character in the matched expression to match
the "extra" ":".  Also checked cris-elf to assert that the expression
still passes for an "ordinary" ELF target.

gcc/testsuite:
	* gcc.dg/tree-ssa/reassoc-20.c: Adjust for mmix.