[llvm] [VPlan] Make canonical IV part of the region (PR #156262)

[via llvm-commits]

================
@@ -870,6 +889,37 @@ void VPRegionBlock::dissolveToCFGLoop() {
   VPBlockUtils::connectBlocks(ExitingLatch, Header);
 }
 
+VPInstruction *VPRegionBlock::getOrCreateCanonicalIVIncrement() {
+  // TODO: Represent the increment as VPRegionValue as well.
+  auto *ExitingLatch = cast<VPBasicBlock>(getExiting());
+  VPValue *CanIV = getCanonicalIV();
+  assert(CanIV && "Expected a canonical IV");
+
+  // Match CanIV + Step or (CanIV + ResumeVal) + Step for epilogue loops.
+  auto m_CanIVIncrement = [CanIV]() {
+    return m_c_Add(
+        m_CombineOr(m_Specific(CanIV), m_c_Add(m_Specific(CanIV), m_LiveIn())),
+        m_VPValue());
+  };
+
+  auto *ExitingTerm = ExitingLatch->getTerminator();
+  VPInstruction *CanIVInc = nullptr;
+  // Try to match BranchOnCount(increment, trip_count) for main loop.
+  if (!match(ExitingTerm,
+             m_BranchOnCount(m_VPInstruction(CanIVInc), m_VPValue()))) {
+    // Try to match BranchOnCond(ICmp EQ(increment, trip_count)) for epilogue.
+    VPValue *Cond = nullptr;
+    if (!(match(ExitingTerm, m_BranchOnCond(m_VPValue(Cond))) &&
+          match(Cond, m_SpecificICmp(CmpInst::ICMP_EQ,
+                                     m_VPInstruction(CanIVInc), m_VPValue())) &&
+          match(CanIVInc, m_CanIVIncrement())))
+      CanIVInc = nullptr;
+  }
+  assert(!CanIVInc || match(CanIVInc, m_CanIVIncrement()) &&
+                          "invalid existing IV increment");
----------------
ayalz wrote:

```suggestion
  // Try to match BranchOnCond(ICmp EQ(increment, trip_count)) for epilogue.
  VPValue *Cond = nullptr;
  if (match(ExitingTerm, m_BranchOnCond(m_VPValue(Cond))) &&
      match(Cond, m_SpecificICmp(CmpInst::ICMP_EQ,
                                 m_VPInstruction(CanIVInc), m_VPValue())) &&
      match(CanIVInc, m_CanIVIncrement()))
    return CanIVInc;
```
where m_CanIVIncrement() is simplified to check epilog case only.

https://github.com/llvm/llvm-project/pull/156262