poly_int: vectorizable_load/store

This patch makes vectorizable_load and vectorizable_store cope with
variable-length vectors.  The reverse and permute cases will be
excluded by the code that checks the permutation mask (although a
patch after the main SVE submission adds support for the reversed
case).  Here we also need to exclude VMAT_ELEMENTWISE and
VMAT_STRIDED_SLP, which split the operation up into a constant
number of constant-sized operations.  We also don't try to extend
the current widening gather/scatter support to variable-length
vectors, since SVE uses a different approach.

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

gcc/
	* tree-vect-stmts.c (get_load_store_type): Treat the number of
	units as polynomial.  Reject VMAT_ELEMENTWISE and VMAT_STRIDED_SLP
	for variable-length vectors.
	(vectorizable_mask_load_store): Treat the number of units as
	polynomial, asserting that it is constant if the condition has
	already been enforced.
	(vectorizable_store, vectorizable_load): Likewise.

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

From-SVN: r256136

gcc/ChangeLog

@@ -2,6 +2,18 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* tree-vect-stmts.c (get_load_store_type): Treat the number of
+	units as polynomial.  Reject VMAT_ELEMENTWISE and VMAT_STRIDED_SLP
+	for variable-length vectors.
+	(vectorizable_mask_load_store): Treat the number of units as
+	polynomial, asserting that it is constant if the condition has
+	already been enforced.
+	(vectorizable_store, vectorizable_load): Likewise.
+
+2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* tree-vect-loop.c (vectorizable_live_operation): Treat the number
 	of units as polynomial.  Punt if we can't tell at compile time
 	which vector contains the final result.