vectorizable_mask_load_store replaces scalar IFN_MASK_LOAD calls with
dummy assignments, so that they never survive vectorisation.  This patch
moves the code to vect_transform_loop instead, so that we only change
the scalar statements once all of them have been vectorised.

This makes it easier to handle other types of functions that need
stubbing out, and also makes it easier to handle groups and patterns.

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

gcc/
	* tree-vect-loop.c (vect_transform_loop): Stub out scalar
	IFN_MASK_LOAD calls here rather than...
	* tree-vect-stmts.c (vectorizable_mask_load_store): ...here.

From-SVN: r256210