fix: update c.nop encoding and assembly to support HINTs (#1177)

[16srivarshitha]

Description

This PR resolves the encoding divergence for c.nop between the unified database and riscv-opcodes, as reported in issue #1177.

Changes

YAML (c.nop.yaml):

Updated the match pattern from the fully constrained value 0x0001 to 000-00000-----01, using wildcards for the immediate field
This change enables the database to correctly recognize c.addi x0, x0, imm (where imm ≠ 0) as c.nop HINT variants

Generator (template_helpers.rb):

Enhanced assembly_fmt and assembly_fmt_args to support dynamic formatting based on the immediate value:

When imm = 0: displays as c.nop
When imm ≠ 0 (HINT variant): displays as c.nop

Verification

• Confirmed the match field correctly enforces the rd = x0 constraint (bits 11-7)
• Verified the Ruby generator properly switches between assembly formats based on the immediate value

Fixes #1177

[ThinkOpenly]

Thank you for this contribution. There are a few issues with it for which I've added comments.

A final implementation is perhaps not possible until optional operands are supported.

If you wish to address the addressable comments, please do. This PR will probably need to stay open for some time, though, and then it will need substantial further work. Shall we leave it open until then, or would it be easier to close it and revisit the issue later?

[ThinkOpenly]

The "assembly" attribute does not include the instruction mnemonic, so this should be changed.

Suggested change

	assembly: "c.nop imm"
	assembly: imm

Also, this is redundant with the "assembly" attribute below. There can be only one.

Also, the "extension" below needs to be under the "definedBy" above, and this was inserted between them.

All that being said, this "imm" operand is optional, and UDB doesn't have a way to specify optional operands at present. Hopefully, this will come with #1527, but that might not be soon.

[ThinkOpenly]

I'd really like to avoid adding special cases for specific instructions in the Ruby code. I suspect this will be easier when operands can be explicitly marked "optional" in the YAML when #1527 is merged.

[16srivarshitha]

@ThinkOpenly Thanks! I understand we need to wait for #1527 to do this properly without special cases in Ruby.

I will remove the Ruby changes and clean up the YAML errors you pointed out. I'll leave this PR open as a draft until the optional operands feature is ready.

[ThinkOpenly]

I think these changes are inadvertent, and need to be reverted?