Don't vectorise single-iteration epilogues

Don't vectorise single-iteration epilogues

With a later patch I saw a case in which we peeled a single iteration
for gaps but didn't need to peel further iterations to make up a full
vector.  We then tried to vectorise the single-iteration epilogue.

2019-11-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vect_analyze_loop): Only try to vectorize
	the epilogue if there are peeled iterations for it to handle.

From-SVN: r277886