tree-optimization/105971 - less surprising refs_may_alias_p_2

tree-optimization/105971 - less surprising refs_may_alias_p_2

When DSE asks whether __real a is using __imag a it gets a surprising
result when a is a FUNCTION_DECL.  The following makes sure this case
is less surprising to callers but keeping the bail-out for the
non-decl case where it is true that PTA doesn't track aliases to code
correctly.

2022-06-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105971
	* tree-ssa-alias.cc (refs_may_alias_p_2): Put bail-out for
	FUNCTION_DECL and LABEL_DECL refs after decl-decl disambiguation
	to leak less surprising alias results.

	* gcc.dg/torture/pr106971.c: New testcase.

(cherry picked from commit 8c2733e16ec1c0cdda3db4cdc5ad158a96a658e8)