[msan] Change logic of ClInstrumentationWithCallThreshold

According to logs, ClInstrumentationWithCallThreshold is workaround
for slow backend with large number of basic blocks.
However, I can't reproduce that one, but I see significant slowdown
after ClCheckConstantShadow. Without ClInstrumentationWithCallThreshold
compiler is able to eliminate many of the branches.

So maybe we should drop ClInstrumentationWithCallThreshold completly.

For now I just change the logic to ignore constant shadow so it will
not trigger callback fallback too early.

Reviewed By: kstoimenov

Differential Revision: https://reviews.llvm.org/D133880