Fix vectorizable_live_operation handling of vector booleans

Fix vectorizable_live_operation handling of vector booleans

vectorizable_live_operation needs to use BIT_FIELD_REF to extract one
element of a vector.  For a packed vector boolean type, the number of
bits to extract should be taken from TYPE_PRECISION rather than TYPE_SIZE.

This is shown by existing tests once SVE is added.

2017-09-18  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_live_operation): Fix element size
	calculation for vector booleans.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r252930