[AArch64] Reject invalid subregs involving partial SVE modes

[AArch64] Reject invalid subregs involving partial SVE modes

When adding partial SVE modes, I'd remembered to handle reloads
in a similar way to full big-endian SVE vectors, but forgot the
just-as-important mode-change rules.

2019-12-19  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_can_change_mode_class):
	Don't allow changes between partial SVE modes and other kinds
	of mode.  Don't allow changes between two partial SVE modes
	if they have different container or element sizes.

gcc/testsuite/
	* gcc.target/aarch64/sve/mixed_size_8.c: New test.

From-SVN: r279572