[llvm] [InstCombine] Added optimisation for trunc (Pow2 >> x) to i1 (PR #157030)

[Yingwei Zheng via llvm-commits]

================
@@ -969,6 +971,26 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
     Changed = true;
   }
 
+  const APInt *C1;
+  Value *V1;
+  // OP = { lshr, ashr }
+  // trunc ( OP i8 C1, V1) to i1 -> icmp eq V1, log_2(C1) iff C1 is power of 2
+  if (DestWidth == 1 && match(Src, m_Shr(m_Power2(C1), m_Value(V1)))) {
+    Value *Right = ConstantInt::get(V1->getType(), C1->countr_zero());
+    Value *Icmp = Builder.CreateICmpEQ(V1, Right);
+    return replaceInstUsesWith(Trunc, Icmp);
+  }
+
+  // OP = { lshr, ashr }
+  // trunc ( OP i8 C1, V1) to i1 -> icmp ult V1, log_2(C1 + 1) iff (C1 + 1) is
+  // power of 2
+  if (DestWidth == 1 && match(Src, m_Shr(m_APInt(C1), m_Value(V1))) &&
+      (*C1 + 1).isPowerOf2()) {
----------------
dtcxzyw wrote:

```suggestion
  if (DestWidth == 1 && match(Src, m_Shr(m_LowBitMask(C1), m_Value(V1)))) {
```

https://github.com/llvm/llvm-project/pull/157030