Remove allocations which are used only for NULL pointer check and free

Remove allocations which are used only for NULL pointer check and free

Extend tree-ssa-dse to remove memory allocations that are used only
to check that return value is non-NULL and freed.

New -fmalloc-dce flag can be used to control malloc/free removal.  I
ended up copying what -fallocation-dse does so -fmalloc-dce=1 enables
malloc/free removal provided return value is unused otherwise and
-fmalloc-dce=2 allows additional NULL pointer checks which it folds to
non-NULL direction.

I also added compensation for the gcc.dg/analyzer/pr101837.c testcase and
added testcase that std::nothrow variant of operator new is now optimized way.

With the -fmalloc-dce=n I can also add a level which emits runtime check for half
of address space and calloc overflow if it seems useful, but perhaps
incrementally.  Adding size parameter tracking is not that hard (I posted WIP
patch for that).

gcc/ChangeLog:

	PR tree-optimization/117370
	* common.opt: Add -fmalloc-dce.
	* common.opt.urls: Update.
	* doc/invoke.texi: Document it; also add missing -flifetime-dse entry.
	* tree-ssa-dce.cc (is_removable_allocation_p): Break out from
	...
	(mark_stmt_if_obviously_necessary): ... here; also check that
	operator new satisfies gimple_call_from_new_or_delete.
	(checks_return_value_of_removable_allocation_p): New Function.
	(mark_all_reaching_defs_necessary_1): add missing case for
	STRDUP and STRNDUP
	(propagate_necessity): Use is_removable_allocation_p and
	checks_return_value_of_removable_allocation_p.
	(eliminate_unnecessary_stmts): Update conditionals that use
	removed allocation; use is_removable_allocation_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cdce3.C: Disable allocation dce.
	* g++.dg/tree-ssa/pr19476-1.C: Likewise.
	* g++.dg/tree-ssa/pr19476-2.C: Likewise.
	* g++.dg/tree-ssa/pr19476-3.C: Likewise.
	* g++.dg/tree-ssa/pr19476-4.C: Likewise.
	* gcc.dg/analyzer/pr101837.c: Disable malloc dce.
	* gcc.dg/tree-ssa/pr19831-3.c: Update.
	* gfortran.dg/pr68078.f90: Disable malloc DCE.