[clang] [llvm] [Clang][LoongArch] Add inline asm support for the `q` constraint (PR #141037)

[WÁNG Xuěruì via cfe-commits]

https://github.com/xen0n commented:

What about making a test case that exerts great pressure on the register allocator to verify that `$ra` does not get accidentally handed out? Or if that's too fragile, if it's possible, make a variable pinned to `$ra` or `$zero` then pass it to the inline asm block with the newly added `q` constraint, to ensure a new temp gets allocated.

https://github.com/llvm/llvm-project/pull/141037