Fix PR c/94726: ICE with __builtin_shuffle and changing of types

Fix PR c/94726: ICE with __builtin_shuffle and changing of types

The problem here is __builtin_shuffle when called with two arguments
instead of 1, uses a SAVE_EXPR to put in for the 1st and 2nd operand
of VEC_PERM_EXPR and when we go and gimplify the SAVE_EXPR, the type
is now error_mark_node and that fails hard.
This fixes the problem by adding a simple check for type of operand
of SAVE_EXPR not to be error_mark_node.

OK? Bootstrapped and tested on aarch64-linux-gnu with no regressions.

gcc/ChangeLog:

	PR c/94726
	* gimplify.c (gimplify_save_expr): Return early
	if the type of val is error_mark_node.

gcc/testsuite/ChangeLog:

	PR c/94726
	* gcc.dg/pr94726.c: New test.