[llvm] [AMDGPU] Save and restore SCC using only 32-bit SGPR. (PR #68367)

[Pierre van Houtryve via llvm-commits]

================
@@ -960,6 +960,19 @@ class SIInstrInfo final : public AMDGPUGenInstrInfo {
 
   unsigned getVALUOp(const MachineInstr &MI) const;
 
+  /// Return true if SCC is deinfed and not dead
----------------
Pierre-vh wrote:

typo `defined`

Also this comment and the ones below could be better. I'd say:
```
\returns true if a non-dead definition of SCC is found between the start of the basic block and \p Before 
```
Same thing for `isSCCUsedAfter`:
```
\returns true if an implicit use of SCC is found between \p After and the end of the basic block
```

https://github.com/llvm/llvm-project/pull/68367