gcc.dg/tree-ssa/forwprop-36.c: Adjust for mmix

gcc.dg/tree-ssa/forwprop-36.c: Adjust for mmix

The label of interest here is "b.0_2" for
e.g. x86_64-pc-linux-gnu but "b::1_2" for mmix-knuth-mmixware.
The test seems to be of some interest for mmix (hey, gcc
open-coded 128-bit integer support behind my back!) so I didn't
want to just disable it.  I also didn't want to obscure the
matching pattern by generalizing it, so instead I added a
separate matching pattern.

gcc/testsuite:
	* gcc.dg/tree-ssa/forwprop-36.c: Adjust for mmix.