AArch64: use different type modifier in arm_neon.td

The 'f' modifier is designed for integer type arguments really (according to
its documentation). It's better to use the "half width, same number" modifier.

Should be no user-visible change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202152 91177308-0d34-0410-b5e6-96231b3b80d8