Fixes HHIR bugs that were causing perflab crashes: guards on
class types and tvBox.
We were ignoring stack guards on class, which was causing the IR to
lose track of types for classes on the stack and causing assertion
failures. Fixed by making class stack type guards into stack type
asserts.
tvBox was not returning the right value, causing VGetS to crash in the
IR.
Changed 1 test and added another to cover VGetS in verify_quick_hhir.
alia