Mark constant-sized objects as addressable if they have poly-int accesses

Mark constant-sized objects as addressable if they have poly-int accesses

If SVE code is written for a specific vector length, it might load from
or store to fixed-sized objects.  This needs to work even without
-msve-vector-bits=N (which should never be needed for correctness).

There's no way of handling a direct poly-int sized reference to a
fixed-size register; it would have to go via memory.  And in that
case it's more efficient to mark the fixed-size object as
addressable from the outset, like we do for array references
with non-constant indices.

2019-12-03  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* cfgexpand.c (discover_nonconstant_array_refs_r): If an access
	with POLY_INT_CST size is made to a fixed-size object, force the
	object to live in memory.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/deref_1.c: New test.

From-SVN: r278941