doloop: Add support for predicated vectorized loops

doloop: Add support for predicated vectorized loops

This patch adds support in the target agnostic doloop pass for the detection of
predicated vectorized hardware loops.  Arm is currently the only target that
will make use of this feature.

gcc/ChangeLog:

	* df-core.cc (df_bb_regno_only_def_find): New helper function.
	* df.h (df_bb_regno_only_def_find): Declare new function.
	* loop-doloop.cc (doloop_condition_get): Add support for detecting
	predicated vectorized hardware loops.
	(doloop_modify): Add support for GTU condition checks.
	(doloop_optimize): Update costing computation to support alterations to
	desc->niter_expr by the backend.

Co-authored-by: Stam Markianos-Wright <stam.markianos-wright@arm.com>