Fix PEELING_FOR_NITERS calculation (PR 87288)

Fix PEELING_FOR_NITERS calculation (PR 87288)

PEELING_FOR_GAPS now means "peel one iteration for the epilogue",
in much the same way that PEELING_FOR_ALIGNMENT > 0 means
"peel that number of iterations for the prologue".  We weren't
taking this into account when deciding whether we needed to peel
further scalar iterations beyond the iterations for "gaps" and
"alignment".

Only the first test failed before the patch.  The other two
are just for completeness.

2018-09-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/87288
	* tree-vect-loop.c (vect_analyze_loop_2): Take PEELING_FOR_GAPS
	into account when determining PEELING_FOR_NITERS.

gcc/testsuite/
	PR tree-optimization/87288
	* gcc.dg/vect/pr87288-1.c: New test.
	* gcc.dg/vect/pr87288-2.c: Likewise,
	* gcc.dg/vect/pr87288-3.c: Likewise.

From-SVN: r264440