[WebAssembly] Support select and block for reference types

This adds missing `select` instruction support and block return type
support for reference types. Also refactors WebAssemblyInstrRef.td and
rearranges tests in reference-types.s. Tests don't include `exnref`
types, because we currently don't support `exnref` for `ref.null` and
the type will be removed soon anyway.

Reviewed By: tlively, sbc100, wingo

Differential Revision: https://reviews.llvm.org/D92359