Improve vectorization COND_EXPR <bool op bool, ...>

Improve vectorization COND_EXPR <bool op bool, ...>

This patch allows us to recognise:

    ... = bool1 != bool2 ? x : y

as equivalent to:

    bool tmp = bool1 ^ bool2;
    ... = tmp ? x : y

For the latter we were already able to find the natural number
of vector units for tmp based on the types that feed bool1 and
bool2, whereas with the former we would simply treat bool1 and
bool2 as vectorised 8-bit values, possibly requiring them to
be packed and unpacked from their natural width.

This is used by a later SVE patch.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-patterns.c (vect_recog_mask_conversion_pattern): When
	handling COND_EXPRs with boolean comparisons, try to find a better
	basis for the mask type than the boolean itself.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256207