aarch64: Fix an ICE in aarch64_short_vector_p [PR95459]

aarch64: Fix an ICE in aarch64_short_vector_p [PR95459]

In aarch64_short_vector_p, we are simply checking whether a type (and a mode)
is a 64/128-bit short vector or not.  This should not be affected by the value
of TARGET_SVE.  Simply leave later code to report an error if SVE is disabled.

2020-06-02  Felix Yang  <felix.yang@huawei.com>

gcc/
	PR target/95459
	* config/aarch64/aarch64.c (aarch64_short_vector_p):
	Leave later code to report an error if SVE is disabled.

gcc/testsuite/
	PR target/95459
	* gcc.target/aarch64/mgeneral-regs_6.c: New test.