[msan] Check mask and pointers shadow

Msan has default handler for unknown instructions which
previously applied to these as well. However depending on
mask, not all pointers or passthru part will be used. This
allows other passes to insert undef into sum arguments.
As result,  default strict instruction handler can produce false reports.

Reviewed By: kda, kstoimenov

Differential Revision: https://reviews.llvm.org/D133678