Record the loop masks needed for EXTRACT_LAST_REDUCTIONs

Record the loop masks needed for EXTRACT_LAST_REDUCTIONs

The analysis phase of vectorizable_condition wasn't recording the
loop masks needed by the transform phase.  This meant that the masks
wouldn't be created in the (rare) case that no other statement needed
them.

2019-12-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_condition): Record the loop
	masks required for extract-last reductions.

gcc/testsuite/
	* gcc.target/aarch64/sve/clastb_9.c: New test.

From-SVN: r279163