[ASAN] Initial support memory checks on scalable vector typed allocas

This patch adjusts the memory instrumentation to account for scalable vector types in allocas. Note that we don't allow scalable vector globals, so we don't need to update that codepath.

A couple points.

First, this simply disables the optimization for scalable allocas. We can revisit this in the future, but it requires a bit of plumbing to get scalable object sizes through the visitor to be useful.

Second, I am simply disabling stack poisoning for scalable vector allocas. This is mostly for staging the change as I can't write a working test for memory instrumentation without doing so. I don't think it's unreasonable to do on it's own basis as without the bailout, we crash the compiler.

Differential Revision: https://reviews.llvm.org/D145259