Split out gather load mask building

Split out gather load mask building

This patch splits out the code to build an all-bits-one or all-bits-zero
input to a gather load.  The catch is that both masks can have
floating-point type, in which case they are implicitly treated in
the same way as an integer bitmask.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-stmts.c (vect_build_all_ones_mask)
	(vect_build_zero_merge_argument): New functions, split out from...
	(vectorizable_load): ...here.

From-SVN: r256214