[hwasan] Do not instrument accesses to uninteresting allocas.

This leads to a statistically significant improvement when using -hwasan-instrument-stack=0: https://bit.ly/3AZUIKI.
When enabling stack instrumentation, the data appears gets better but not statistically significantly so. This is consistent
with the very moderate improvements I have seen for stack safety otherwise, so I expect it to improve when the underlying
issue of that is resolved.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D108457