[WebAssembly] Serialize params/results in MachineFunctionInfo

This adds support for YAML serialization of `Params` and `Results`
fields in `WebAssemblyMachineFunctionInfo`. Types are printed as `MVT`'s
string representation. This is for writing MIR tests easier.

The tests added are testing simple parsing and printing of `params` /
`results` fields under `machineFunctionInfo`.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D101029