expand_call_mem_ref checks for TARGET_MEM_REFs that have compatible
type, but it didn't then go on to install the specific type we need,
which might have different alignment due to:

  if (TYPE_ALIGN (type) != align)
    type = build_aligned_type (type, align);

This was causing masked stores to be incorrectly marked as
aligned on AVX512.

2018-02-20  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/84419
	* internal-fn.c (expand_call_mem_ref): Create a TARGET_MEM_REF
	with the required type if its current type is compatible but
	different.

gcc/testsuite/
	PR tree-optimization/84419
	* gcc.dg/vect/pr84419.c: New test.

From-SVN: r257847