[WebAssembly] Use ArrayRef/SmallVectorImpl in parameters (NFC)

It is recommended to use `SmallVectorImpl`/`ArrayRef` over
`SmallVector<TypeName, N>` for function parameters:
https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D146841