AArch64: Fix vector re-interpretation between partial SIMD modes

While writing a patch series I started getting incorrect codegen out from
VEC_PERM on partial struct types.

It turns out that this was happening because the TARGET_CAN_CHANGE_MODE_CLASS
implementation has a slight bug in it.  The hook only checked for SIMD to
Partial but never Partial to SIMD.   This resulted in incorrect subregs to be
generated from the fallback code in VEC_PERM_EXPR expansions.

I have unfortunately not been able to trigger it using a standalone testcase as
the mid-end optimizes away the permute every time I try to describe a permute
that would result in the bug.

The patch now rejects any conversion of partial SIMD struct types, unless they
are both partial structures of the same number of registers or one is a SIMD
type who's size is less than 8 bytes.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_can_change_mode_class): Restrict
	conversions between partial struct types properly.