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

Track the live register state immediately before, instead of after,
MBBI. This makes it simple to track the state at the start or end of a
basic block without a separate (and poorly named) Tracking flag.

This changes the API of the backward(MachineBasicBlock::iterator I)
method, which now recedes to the state just before, instead of just
after, *I. Some clients are simplified by this change.

There is one small functional change shown in the lit tests where
multiple spilled registers all need to be reloaded before the same
instruction. The reloads will now be inserted in the opposite order.
This should not affect correctness.

1 files changed, 1 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/PrologEpilogInserter.cpp b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
index 0b8dcdc..8af17e6 100644
--- a/llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -1499,7 +1499,7 @@ void PEI::replaceFrameIndicesBackward(MachineBasicBlock *BB,
 
     // Step backwards to get the liveness state at (immedately after) MI.
     if (LocalRS)
-      LocalRS->backward(MI);
+      LocalRS->backward(I);
 
     bool RemovedMI = false;
     for (const auto &[Idx, Op] : enumerate(MI.operands())) {
@@ -1515,11 +1515,6 @@ void PEI::replaceFrameIndicesBackward(MachineBasicBlock *BB,
         break;
     }
 
-    // Refresh the scavenger's internal iterator in case MI was removed or more
-    // instructions were inserted after it.
-    if (LocalRS)
-      LocalRS->skipTo(std::prev(I));
-
     if (!RemovedMI)
       --I;
   }