Like the entry block arguments, the return values from a call
instruction need to be converted back from their ABI representation.
Add tests of call instruction legalization.
- abi.cton is for testing the actual RISC-V ABI.
- legalize-abi.cton is for testing the legalizer around ABI boundaries.
- parse-encoding.cton is for testing the parser's handling of RISC-V
encoding and register annotations.