[InstCombine] Refactor foldSelectICmpAndOr to use `decomposeBitTestICmp` instead of bespoke logic

[goldstein.w.n]

Summary

This is essentially NFC as the cases decomposeBitTestICmp covers
that weren't already covered explicitly, will be canonicalized into
the cases explicitly covered. As well the unsigned cases don't apply
as the Mask is not a power of 2.

That being said, using a well established helper is less bug prone and
if some canonicalization changes, will prevent regressions here.

[goldstein.w.n]

Rebase

[nikic]

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

692

Probably better to pass CmpLHS instead of Unused? They'll always be the same without LookThruTrunc, but that seems more robust.

697

I don't really get why this isn't NeedAnd = true any more.

[goldstein.w.n]

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

697

Can't remember why I changed that. Probably was a mistake either way.

[goldstein.w.n]

Remove Unused variable and remove change to NeedAnd logic

[goldstein.w.n]

Rebase

[nikic]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

static Value *foldSelectICmpAndOr(const ICmpInst *IC, Value *TrueVal,
   if (!TrueVal->getType()->isIntOrIntVectorTy() ||
       TrueVal->getType()->isVectorTy() != IC->getType()->isVectorTy())
     return nullptr;
 
   Value *CmpLHS = IC->getOperand(0);
   Value *CmpRHS = IC->getOperand(1);
 
   unsigned C1Log;
-  bool IsEqualZero;
   bool NeedAnd = false;
+  CmpInst::Predicate Pred = IC->getPredicate();
   if (IC->isEquality()) {
     if (!match(CmpRHS, m_Zero()))
       return nullptr;
 
     const APInt *C1;
     if (!match(CmpLHS, m_And(m_Value(), m_Power2(C1))))
       return nullptr;
 
     C1Log = C1->logBase2();
-    IsEqualZero = IC->getPredicate() == ICmpInst::ICMP_EQ;
   } else {
-    // We also need to recognize (icmp slt X, 0) and (icmp sgt X, -1).
-    if (IC->getPredicate() == ICmpInst::ICMP_SGT && match(CmpRHS, m_AllOnes()))
-      IsEqualZero = true;
-    if (IC->getPredicate() == ICmpInst::ICMP_SLT && match(CmpRHS, m_Zero()))
-      IsEqualZero = false;
-    else
+    Value *Unused;
+    APInt C1;
+    if (!decomposeBitTestICmp(CmpLHS, CmpRHS, Pred, Unused, C1) ||
+        !C1.isPowerOf2())
       return nullptr;
 
-    C1Log = CmpLHS->getType()->getScalarSizeInBits() - 1;
-    NeedAnd = true;
+    C1Log = C1.logBase2();
+    NeedAnd = !match(CmpRHS, m_SpecificInt(C1));
   }
 
   Value *Or, *Y, *V = CmpLHS;
   const APInt *C2;
   bool NeedXor;
   if (match(FalseVal, m_Or(m_Specific(TrueVal), m_Power2(C2)))) {
     Y = TrueVal;
     Or = FalseVal;
-    NeedXor = !IsEqualZero;
+    NeedXor = Pred == ICmpInst::ICMP_NE;
   } else if (match(TrueVal, m_Or(m_Specific(FalseVal), m_Power2(C2)))) {
     Y = FalseVal;
     Or = TrueVal;
-    NeedXor = IsEqualZero;
+    NeedXor = Pred == ICmpInst::ICMP_EQ;
   } else {
     return nullptr;
   }
 
   unsigned C2Log = C2->logBase2();
 
   bool NeedShift = C1Log != C2Log;
   bool NeedZExtTrunc = Y->getType()->getScalarSizeInBits() !=