[CodeGen] Use SmallVector for FixedStackPSVs (#91760)

Frame indices are dense and consecutive, so use a vector instead of a
std::map. Due to possibly negative frame indices, use zig-zag encoding.
IndexedMap was not usable, as it attempted to copy the null value, which
is not possible with a std::unique_ptr.

This is just a minor performance improvement, but a low-hanging fruit.