[llvm] [LLVM] Improve the DemandedBits Analysis (PR #148853)

[Yingwei Zheng via llvm-commits]

================
@@ -246,6 +247,59 @@ void DemandedBits::determineLiveOperandBits(
     else
       AB &= ~(Known.One & ~Known2.One);
     break;
+  case Instruction::SRem:
+  case Instruction::URem:
+  case Instruction::UDiv:
+  case Instruction::SDiv: {
+    auto Opc = UserI->getOpcode();
+    auto IsDiv = Opc == Instruction::UDiv || Opc == Instruction::SDiv;
+    bool IsSigned = Opc == Instruction::SDiv || Opc == Instruction::SRem;
+    if (OperandNo == 0) {
+      const APInt *DivAmnt;
+      if (match(UserI->getOperand(1), m_APInt(DivAmnt))) {
+        uint64_t D = DivAmnt->getZExtValue();
+        if (isPowerOf2_64(D)) {
+          unsigned Sh = Log2_64(D);
+          if (IsDiv) {
+            AB = AOut.shl(Sh);
+          } else {
+            AB = AOut & APInt::getLowBitsSet(BitWidth, Sh);
+          }
+        } else if (IsDiv) { // Non power of 2 constant div
+          //   x =  q * C + r;
+          //   q = x / C;
+          //   We think of it like grade school division in base 2.
+          //
+          //    x = [   unused   |  window m-bits |  ...  | needed bits ]
+          //                         ^ each step emits 1 quotient bit
+          //                         |
+          //                         |
+          //   C fits in m = ⌈log₂ C⌉ bits
----------------
dtcxzyw wrote:

```suggestion
          //   C fits in m = ⌈log2 C⌉ bits
```
Avoid non-ASCII characters.

https://github.com/llvm/llvm-project/pull/148853