[AArch64] Tighten aarch64_secondary_reload condition (PR 83845)

[AArch64] Tighten aarch64_secondary_reload condition (PR 83845)

aarch64_secondary_reload enforced a secondary reload via
aarch64_sve_reload_be for memory and pseudo registers, but failed
to do the same for subregs of pseudo registers.  To avoid this and
any similar problems, the patch instead tests for things that the move
patterns handle directly; if the operand isn't one of those, we should
use the reload pattern instead.

The patch fixes an ICE in sve/mask_struct_store_3.c for aarch64_be,
where the bogus target description was (rightly) causing LRA to cycle.

2018-02-01  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tearget/83845
	* config/aarch64/aarch64.c (aarch64_secondary_reload): Tighten
	check for operands that need to go through aarch64_sve_reload_be.

Reviewed-by: James Greenhalgh <james.greenhalgh@arm.com>
From-SVN: r257285