[RISCV] Backport TableGen-based macro fusion changes

[wangpc-pp]

/cherry-pick 664a0fa 3fdb431

Backport these changes into LLVM 18 so that we will have a released baseline that supports TableGen-based macro fusion.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: Wang Pengcheng (wangpc-pp)

/cherry-pick 664a0fa 3fdb431

Backport these changes into LLVM 18 so that we will have a released baseline that supports TableGen-based macro fusion.

[github-actions]

/branch llvmbot/llvm-project/issue79425

[github-actions]

/branch llvmbot/llvm-project/issue79425

[nikic]

PR manually opened at #79560.

[tstellar]

Merged: d9e26c2 7cfa0c1