RISC-V: fix C.Nop parsing

What

Fixes a bug in the compressed_instruction parser where some HINTS were incorrectly decoded as C.Nop.

Why

Correctness

How

Found by the included test: namely, that parsing an instruction and then converting it back to the original should be the identity function (ie that every possible u16 corresponds to a singular & unique result from parse_compressed

Manually Testing

make -C src/riscv all

Benchmarking

This change has been benchmarked on the benchmark machine and does not affect performance

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
• Benchmark performance and populate the table above if needed.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.

/assign @ole.kruger
/assign @victor-dumitrescu
/assign @felix.puscasu1
/assign @anastasia.courtney
/assign @emturner

/assign_reviewer @ole.kruger
/assign_reviewer @victor-dumitrescu
/assign_reviewer @felix.puscasu1
/assign_reviewer @anastasia.courtney
/assign_reviewer @emturner

/unassign me
/unassign_reviewer me

/ready