middle-end: Fix phi-ssa assertion triggers. [PR106519]

For the diamond PHI form in tree_ssa_phiopt_worker we need to
extract edge e2 sooner.  This changes it so we extract it at the
same time we determine we have a diamond shape.

gcc/ChangeLog:

	PR middle-end/106519
	* tree-ssa-phiopt.cc (tree_ssa_phiopt_worker): Check final phi edge for
	diamond shapes.

gcc/testsuite/ChangeLog:

	PR middle-end/106519
	* gcc.dg/pr106519.c: New test.