tree-optimization/109143 - improve PTA compile time

tree-optimization/109143 - improve PTA compile time

The following improves solution_set_expand to require one less
iteration over the bitmap and avoid changing the bitmap we iterate
over.  Plus we handle adjacent subvars in the ID space (the common case)
and use bitmap_set_range.  This cuts a bit less than 10% off the PTA
time from the testcase in the PR.

	PR tree-optimization/109143
	* tree-ssa-structalias.cc (solution_set_expand): Avoid
	one bitmap iteration and optimize bit range setting.