[msan] Set cmpxchg shadow precisely

In terms of https://llvm.org/docs/LangRef.html#cmpxchg-instruction,
the return type of chmpxchg is a pair {ty, i1}, while I think we
only wanted to set the shadow for the address 0th op, and it has type
ty.

Reviewed-by: eugenis

Differential Revision: https://reviews.llvm.org/D97029