The IFN_GOMP_SIMD_LANE handling in vectorizable_store tries to use MEM_REF
offsets to maintain pointer disambiguation info.  This patch makes sure
that we don't try to do the same optimisation for IFN_MASK_STOREs, which
have no similar offset argument.

The patch fixes libgomp.c-c++-common/pr66199-*.c for SVE.  Previously
we had an ncopies==2 store and stored both halves to the same address.

2019-01-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_store): Don't use the dataref_offset
	optimization for masked stores.

From-SVN: r267654