[llvm] [MIPS]Initial support for MIPS16 assembly. (PR #108681)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The MIPS port lacks adequate maintenance and has accumulated significant technical debt. Few people seem to care about it today.

I maintain MC and binary utilities, occasionally contributing to or reviewing MIPS changes.
To be clear, I have little interest in MIPS, but I’m concerned about its impact on specific components like the Clang driver, ld.lld, MC, and compiler-rt.
https://maskray.me/blog/2023-09-04-toolchain-notes-on-mips

Regarding this PR: MIPS16 and microMIPS are already partially supported (likely with many incorrect things), and since this PR modifies codegen, the title "[MIPS] Initial support for MIPS16 assembly" may be misleading (note: we omit periods in subject lines).

I doubt any regular contributor uses MIPS16 or microMIPS, so finding someone to thoroughly review these functional changes could be challenging.
I can provide a rubber stamp if needed.

The llvm-project receives over 100 commits daily. After a PR is approved, it’s best to rebase and retrigger CI if the PR has been stale for a few days.

https://github.com/llvm/llvm-project/pull/108681