[AArch64][NEON] Remove undocumented vceqz{,q}_p16, vml{a,s}q_n_f64 intrinsics

Prior to this patch, Clang supported the following C/C++ intrinsics:

    vceqz_p16
    vceqzq_p16
    vmlaq_n_f64
    vmlsq_n_f64

... exposed through arm_neon.h. However, these intrinsics are not part
of the ACLE, allowing developers to write code that is not compatible
with other toolchains.

This patch removes these intrinsics.

There is a bug report capturing this issue here:

    https://bugs.llvm.org/show_bug.cgi?id=47471

Reviewed By: bsmith

Differential Revision: https://reviews.llvm.org/D93206