Fix stmt_kills_ref_p WRT external throws

Fix stmt_kills_ref_p WRT external throws

Add missing check to stmt_kills_ref_p for case that function
is terminated by EH before call return value kills the ref. In the PR
I tried to construct testcase but I don't know how to do that until I
annotate EH code with fnspec attributes which I will do in separate patch
and add a testcase.

	PR ipa/106057
	* tree-ssa-alias.cc (stmt_kills_ref_p): Check for external throw.

(cherry picked from commit 7fd34782b95bbe1b4dc9936b8923f86d4aaee379)