[AArch64] Use CMTST for != 0 vector compares (vnot (CMEQz A)).

(CMTST A, A) will only set elements to 0 if the element is 0 in A. Use
it for != 0 compares, which currently use (vnot (CMEQz A)). This saves a
mvn instruction.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D97303