Restore flow_bb_inside_loop_p tests (PR 86858)

Restore flow_bb_inside_loop_p tests (PR 86858)

The series to remove vinfo_for_stmt also removed tests of
flow_bb_inside_loop_p if the call was simply testing whether the
statement was in the vectorisation region.  I'd tried to keep calls
that were testing whether the statement was in a particular loop
(inner or outer), but messed up in vect_is_simple_reduction and
removed calls that were actually needed.  This patch restores them.

I double-checked the other removed calls and I think these are
the only ones affected.

2018-08-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86858
	* tree-vect-loop.c (vect_is_simple_reduction): Restore
	flow_bb_inside_loop_p calls.

gcc/testsuite/
	PR tree-optimization/86858
	* gcc.dg/vect/pr86858.c: New test.

From-SVN: r263448