Zicfiss instruction encoding updates

[ved-rivos]

The ARC has realized that only a subset of SS instructions really need to be mops. Currently, these are SSRDP, SSPUSH, and SSPOPCHK. The other SS instructions should have non-mop encodings. This issue is opened to track the exact encodings that are pending allocation.

[ved-rivos]

The ssamoswap is the one instruction to be encoded using a non-mop code point. The direction is to use a funct5 encoding of the AMO opcode for this instruction. Update pending allocation of the encoding.

[ved-rivos]

PR #159 created to update the code points.

• sspush updated to use mop.rr.7, which decodes as similarly as possible to HSV.D
• sspopchk updated to use mop.r.28, which decodes as similarly as possible to HLV.D
• ssrdp updated to use mop.r.0

[ved-rivos]

Closing this issue. Please leave feedback in the PR.

[ved-rivos]

Couple of additional updates:

1. SSRDP moved to use MOP.R.28
2. The Zcmop mnemonics were changed to C.MOP.1, C.MOP.3, C.MOP.5, ... -> with this update the code points used by Zicfiss would have mnemonics C.MOP.1 and C.MOP.5.