tree-optimization/117510 - fix guard hoisting validity check

tree-optimization/117510 - fix guard hoisting validity check

For the loop in the testcase we currently fail to hoist the guard
check of the inner loop (m > 0) out of the outer loop because
find_loop_guard checks all blocks of the outer loop for side-effects,
including those that are skipped by the guard.  This usually
is harmless as the guard does not skip any blocks in the outer loop
but in this case store-motion was applied to the inner loop and thus
there's now a skipped store in the outer loop.

The following properly skips blocks that are dominated by the
entry to the skipped region.

	PR tree-optimization/117510
	* tree-ssa-loop-unswitch.cc (find_loop_guard): Only check
	not skipped blocks for side-effects.

	* gcc.dg/vect/vect-outer-pr117510.c: New testcase.