Fix PR 106560: Another ICE after conflicting types of redeclaration

This another one of these ICE after error issues with the
gimplifier and a fallout from r12-3278-g823685221de986af.
The problem here is gimplify_modify_expr does not
check if either from or to was an error operand.
This adds the check and fixes the ICE.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* gimplify.cc (gimplify_modify_expr): If
	either *from_p or *to_p were error_operand
	return early.

gcc/testsuite/ChangeLog:

	* gcc.dg/redecl-23.c: New test.
	* gcc.dg/redecl-24.c: New test.
	* gcc.dg/redecl-25.c: New test.