[MSAN RT] Use __sanitizer::mem_is_zero in __msan_test_shadow

The former function is particularly optimized for exactly the
use case we're interested in: an all-zero buffer.

This reduces the overhead of calling this function some 80% or
more. This is particularly for instrumenting code heavy with
string processing functions, like grep. An invocation of grep
with the pattern '[aeiou]k[aeiou]' has its runtime reduced by
~75% with this patch

Differential Revision: https://reviews.llvm.org/D84961