[llvm] [RegScavenger] Simplify state tracking for backwards scavenging (PR #71202)

[Jay Foad via llvm-commits]

================
@@ -466,7 +466,7 @@ PPCFrameLowering::findScratchRegister(MachineBasicBlock *MBB,
       RS.enterBasicBlock(*MBB);
----------------
jayfoad wrote:

Same here, the `MBBI == MBB->begin()` case is not required.

https://github.com/llvm/llvm-project/pull/71202