Restore correct iv step for fully-masked loops

Restore correct iv step for fully-masked loops

r272233 introduced a large number of execution failures on SVE.
The patch hard-coded an IV step of VF, but for SLP groups it needs
to be VF * group size.

Also, iv_precision had type widest_int but only needs to be unsigned int.

2019-06-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop-manip.c (vect_set_loop_masks_directly): Remove
	vf parameter.  Restore the previous iv step of nscalars_step,
	but give it iv_type rather than compare_type.  Tweak code order
	to match the comments.
	(vect_set_loop_condition_masked): Update accordingly.
	* tree-vect-loop.c (vect_verify_full_masking): Use "unsigned int"
	for iv_precision.  Tweak comment formatting.

From-SVN: r272411