Avoid -Wuninitialized false negatives with sanitization and VLAs.

Avoid -Wuninitialized false negatives with sanitization and VLAs.

Resolves:
PR tree-optimization/93100 - gcc -fsanitize=address inhibits -Wuninitialized
PR middle-end/98583 - missing -Wuninitialized reading from a second VLA in its own block

gcc/ChangeLog:

	PR tree-optimization/93100
	PR middle-end/98583
	* tree-ssa-uninit.c (check_defs): Exclude intrinsic functions that
	don't modify referenced objects.

gcc/testsuite/ChangeLog:

	PR tree-optimization/93100
	PR middle-end/98583
	* g++.dg/warn/uninit-pr93100.C: New test.
	* gcc.dg/uninit-pr93100.c: New test.
	* gcc.dg/uninit-pr98583.c: New test.