tree-optimization/105801 - CCP and .DEFERRED_INIT

tree-optimization/105801 - CCP and .DEFERRED_INIT

This makes sure we treat .DEFERRED_INIT as producing UNDEFINED so
we can continue optimizing uninitialized uses the same as without
-ftrivial-auto-var-init=zero.  For the testcase this means we
catch the return 1 optimization opportunity at CCP rather than
only at FRE which already does the right thing here.

	PR tree-optimization/105801
	* tree-ssa-ccp.cc (likely_value): .DEFERRED_INIT produces
	UNDEFINED.
	* doc/invoke.texi (ftrivial-auto-var-init): Explicitely
	mention we treat variables without an initializer as
	undefined also for optimization purposes.

	* gcc.dg/tree-ssa/ssa-ccp-43.c: New testcase.