tree-optimization/110799 - fix bug in code hoisting

Code hoisting part of GIMPLE PRE failed to adjust the TBAA behavior
of common loads in the case the alias set of the ref was the same
but the base alias set was not.  It also failed to adjust the
base behavior, assuming it would match.  The following plugs this
hole.

	PR tree-optimization/110799
	* tree-ssa-pre.cc (compute_avail): More thoroughly match
	up TBAA behavior of redundant loads.

	* gcc.dg/torture/pr110799.c: New testcase.