forwprop: Allow mismatch clobbers in simple dse

forwprop: Allow mismatch clobbers in simple dse

As mention in the patch that adds DSEing lhs of calls,
some testcases were xfailed due to exceptions and mismatch
of clobbers in some cases.
This allows them and un-xfails the testcase where they show
up.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (do_simple_agr_dse): Allow
	for mismatched clobbers.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/simple-dse-3.C: un-xfail.

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