forwprop: Handle memcpy for arguments with respect to copies

forwprop: Handle memcpy for arguments with respect to copies

This moves the code used in optimize_agr_copyprop_1 (r16-3887-g597b50abb0d)
to handle this same case into its new function and use it inside
optimize_agr_copyprop_arg. This allows to remove more copies that show up only
in arguments.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (optimize_agr_copyprop_1): Split out
	the case where `operand_equal_p (dest, src2)` is false into ...
	(new_src_based_on_copy): This. New function.
	(optimize_agr_copyprop_arg): Use new_src_based_on_copy
	instead of operand_equal_p to find the new src.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/copy-prop-aggregate-arg-2.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>