cfgexpand: Factor out getting the stack decl index

This is the first patch in improving this code.
Since there are a few places which get the index and they
check the same thing, let's factor that out into one function.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* cfgexpand.cc (INVALID_STACK_INDEX): New defined.
	(decl_stack_index): New function.
	(visit_op): Use decl_stack_index.
	(visit_conflict): Likewise.
	(add_scope_conflicts_1): Likewise.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>