c++: TARGET_EXPR collapsing [PR107303]

c++: TARGET_EXPR collapsing [PR107303]

In r13-2978 I tried to eliminate TARGET_EXPR around TARGET_EXPR by
discarding the outer one, but as in this testcase that breaks if the
TARGET_EXPR_SLOT of the outer one is used elsewhere.  But it should always
be safe to strip the inner one; if its slot were reused, there would be a
COMPOUND_EXPR around the TARGET_EXPR.

For 107329, if we're setting *walk_subtrees, we also need to fold
TARGET_EXPR_CLEANUP.

	PR c++/107303
	PR c++/107329

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_r) [TARGET_EXPR]: In case of double
	TARGET_EXPR, keep the outer one instead of the inner one.
	(maybe_replace_decl): New.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/builtin-shufflevector-5.C: New test.
	* g++.dg/init/new51.C: New test.