[AArch64] Tighten mode checks in aarch64_builtin_vectorized_function

[AArch64] Tighten mode checks in aarch64_builtin_vectorized_function

aarch64_builtin_vectorized_function checked vectors based on the
number of elements and the element mode.  This doesn't interact
well with fixed-length 128-bit SVE, where SVE modes can have those
same properties.  (And we can't just use the built-ins for SVE because
the types use a different ABI.  SVE handles this kind of thing using
optabs instead.)

2020-01-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-builtins.c
	(aarch64_builtin_vectorized_function): Check for specific vector modes,
	rather than checking the number of elements and the element mode.

From-SVN: r280114