[llvm] [DemandedBits] Support non-constant shift amounts (PR #148880)

[Panagiotis Karouzakis via llvm-commits]

================
@@ -76,6 +76,19 @@ void DemandedBits::determineLiveOperandBits(
           computeKnownBits(V2, Known2, DL, &AC, UserI, &DT);
         }
       };
+  auto GetShiftedRange = [&](uint64_t Min, uint64_t Max, bool ShiftLeft) {
+    auto ShiftF = [ShiftLeft](const APInt &Mask, unsigned ShiftAmnt) {
+      return ShiftLeft ? Mask.shl(ShiftAmnt) : Mask.lshr(ShiftAmnt);
+    };
+    AB = APInt::getZero(BitWidth);
+    uint64_t LoopRange = Max - Min;
+    APInt Mask = AOut;
+    for (unsigned ShiftAmnt = 1; ShiftAmnt <= LoopRange; ShiftAmnt <<= 1) {
----------------
karouzakisp wrote:

I meant this
  `  Known = Known.intersectWith(ShiftByConst(LHS, ShiftAmt)); `
  Anyway, since we are using APInts if we switch the loop to do increments by 1, even if we employ the same strategy as the KnownBits technique the time complexity is O(n^2) instead of O(nlogn)

https://github.com/llvm/llvm-project/pull/148880