gimple: Simplify gimple_seq_nondebug_singleton_p

The implementation of gimple_seq_nondebug_singleton_p
was convoluted on how to determine if the sequence
was a singleton (which could contain debug statements).

This simplifies the function into two calls. One to get the start
after all of the debug statements and then check to see if it
is at the one before the end (or there is only debug statements
afterwards).

Bootstrapped and tested on x86_64-linux-gnu (including ada).

gcc/ChangeLog:

	* gimple-iterator.h (gimple_seq_nondebug_singleton_p):
	Rewrite to be simplely, gsi_start_nondebug/gsi_one_nondebug_before_end_p.

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