[llvm] [InstCombine] Prefer to keep power-of-2 constants when combining ashr exact and slt/ult of a constant (PR #86111)

[Yingwei Zheng via llvm-commits]

================
@@ -2482,10 +2482,21 @@ Instruction *InstCombinerImpl::foldICmpShrConstant(ICmpInst &Cmp,
   // those conditions rather than checking them. This is difficult because of
   // undef/poison (PR34838).
   if (IsAShr && Shr->hasOneUse()) {
+    if (IsExact && (Pred == CmpInst::ICMP_SLT || Pred == CmpInst::ICMP_ULT) &&
+        !C.isMinSignedValue() && (C - 1).isPowerOf2()) {
+      // When C - 1 is a power of two and the transform can be legally
+      // performed, prefer this form so the produced constant is close to a
+      // power of two.
+      // icmp slt/ult (ashr exact X, ShAmtC), C
+      // --> icmp slt/ult (C - 1) << ShAmtC) -1
----------------
dtcxzyw wrote:

```suggestion
      // --> icmp slt/ult X, (C - 1) << ShAmtC) + 1
```

https://github.com/llvm/llvm-project/pull/86111