[AArch64] Fix label mode

[AArch64] Fix label mode

This patch fixes a case where we tried to force a VOIDmode label
into a DImode register, which led to invalid rtl that was previously
undiagnosed.

2017-08-22  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* config/aarch64/aarch64.md (casesi): Use DImode rather than
	VOIDmode for the LABEL_REF.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251266