[AArch64] Reject (high (const (plus anchor offset)))

[AArch64] Reject (high (const (plus anchor offset)))

The aarch64_legitimate_constant_p tests for HIGH and CONST seem
to be the wrong way round: (high (const ...)) is valid rtl that
could be passed in, but (const (high ...)) isn't.  As it stands,
we disallow anchor+offset but allow (high anchor+offset).

2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p): Fix
	order of HIGH and CONST checks.

From-SVN: r256384