Bug 1695778 - (Core-Wasm-EH-Ion) part 4: Support reftypes in exceptions. r=rhunt

Bug 1695778 - (Core-Wasm-EH-Ion) part 4: Support reftypes in exceptions. r=rhunt

This patch concludes support for the WebAssembly Exception Handling proposal.

Spec: https://github.com/WebAssembly/exception-handling

Changes:

- Mirroring the Baseline approach, and the Ion approach for numeric values,
  this patch adds two MIR and LIR nodes:
  - WasmExceptionRefsPointer to get a pointer to the ArrayObject REFS of a
    WasmExceptionObject, and
  - WasmLoadExceptionRefsValue to get a RefOrNull value from a
    WasmExceptionRefsPointer
- Storing a RefOrNull value in the exception is done via the instance call
  SASigPushRefIntoExn. Since this pushes the value in the Refs Array, we
  iterate backwards.
- The above mentioned instance call is changed to follow this review
  comment on part 1 of this patchset:
  https://phabricator.services.mozilla.com/D118283#inline-716841
- Enables all tests in .../wasm/exceptions/, and adds reftypes in some
  existing tests.

Differential Revision: https://phabricator.services.mozilla.com/D132637