tree-optimization/111715 - improve TBAA for access paths with pun

tree-optimization/111715 - improve TBAA for access paths with pun

The following improves basic TBAA for access paths formed by
C++ abstraction where we are able to combine a path from an
address-taking operation with a path based on that access using
a pun to avoid memory access semantics on the address-taking part.

The trick is to identify the point the semantic memory access path
starts which allows us to use the alias set of the outermost access
instead of only that of the base of this path.

	PR tree-optimization/111715
	* alias.cc (reference_alias_ptr_type_1): When we have
	a type-punning ref at the base search for the access
	path part that's still semantically valid.

	* gcc.dg/tree-ssa/ssa-fre-102.c: New testcase.