[msan] Instrument vtest instrinsics

Instrumentation just ORs shadow of inputs.
I assume some result shadow bits can be reset if we go into specifics of particular checks,
but as-is it is still an improvement against existing default strict instruction handler, when
every set bit of input shadow is  reported as an error.

Reviewed By: kda

Differential Revision: https://reviews.llvm.org/D134123