[WebAssembly] Improve pseudocode in LowerEmscriptenEHSjLj

Both `__THREW__` and `__threwValue` are global variables, and we have
been distinguishing the global variable `__THREW__` and the loaded value
`%__THREW__.val` in comments but not doing it for `__threwValue`. Made
the pseudocode comments consistent for both variables.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D106524