DCE __cxa_atexit calls where the function is pure/const [PR19661]

DCE __cxa_atexit calls where the function is pure/const [PR19661]

In C++ sometimes you have a deconstructor function which is "empty", like for an
example with unions or with arrays.  The front-end might not know it is empty either
so this should be done on during optimization.o
To implement it I added it to DCE where we mark if a statement is necessary or not.

Bootstrapped and tested on x86_64-linux-gnu with no regressions.

Changes since v1:
  * v2: Add support for __aeabi_atexit for arm-*eabi. Add extra comments.
        Add cxa_atexit-5.C testcase for -fPIC case.
  * v3: Fix testcases for the __aeabi_atexit (forgot to do in the v2).

	PR tree-optimization/19661

gcc/ChangeLog:

	* tree-ssa-dce.cc (is_cxa_atexit): New function.
	(is_removable_cxa_atexit_call): New function.
	(mark_stmt_if_obviously_necessary): Don't mark removable
	cxa_at_exit calls.
	(mark_all_reaching_defs_necessary_1): Likewise.
	(propagate_necessity): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/cxa_atexit-1.C: New test.
	* g++.dg/tree-ssa/cxa_atexit-2.C: New test.
	* g++.dg/tree-ssa/cxa_atexit-3.C: New test.
	* g++.dg/tree-ssa/cxa_atexit-4.C: New test.
	* g++.dg/tree-ssa/cxa_atexit-5.C: New test.
	* g++.dg/tree-ssa/cxa_atexit-6.C: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>