idiom_analysis: Disable idiom AShr(X,shift) <-> SDiv(X,shift) (#724)

Closes #724

Disables idiom that exchanged Pattern AShr(X,shift) into divistion
SDiv(X, shift). This is because signed division is not equal to the
arithmetic shift right all the time. Only when X is positive integer.

Example:

* AShr(0xA5A5A5A5A5A50020, 8) = 0xFFA5A5A5A5A5A500
* SDiv(0xA5A5A5A5A5A50020, 256) = 0xFFA5A5A5A5A5A501

include/retdec/bin2llvmir/optimizations/idioms/idioms_common.h

@@ -23,7 +23,6 @@ class IdiomsCommon: virtual public IdiomsAbstract {
 	llvm::Instruction * exchangeLessThanZero(llvm::BasicBlock::iterator iter) const;
 	llvm::Instruction * exchangeGreaterEqualZero(llvm::BasicBlock::iterator iter) const;
 	llvm::Instruction * exchangeBitShiftSDiv1(llvm::BasicBlock::iterator iter) const;
-	llvm::Instruction * exchangeBitShiftSDiv2(llvm::BasicBlock::iterator iter) const;
 	llvm::Instruction * exchangeBitShiftUDiv(llvm::BasicBlock::iterator iter) const;
 	llvm::Instruction * exchangeBitShiftMul(llvm::BasicBlock::iterator iter) const;
 	llvm::Instruction * exchangeSignedModulo2n(llvm::BasicBlock::iterator iter) const;

src/bin2llvmir/optimizations/idioms/idioms_analysis.cpp

@@ -226,11 +226,6 @@ bool IdiomsAnalysis::doAnalysis(Function & f, Pass * p) {
 		change_made |= analyse(bb, &IdiomsCommon::exchangeBitShiftSDiv1,
 									"IdiomsCommon::exchangeBitShiftSDiv1");
 
-		// all arch
-		// all compilers
-		change_made |= analyse(bb, &IdiomsCommon::exchangeBitShiftSDiv2,
-									"IdiomsCommon::exchangeBitShiftSDiv2");
-
 		// all arch
 		// all compilers
 		change_made |= analyse(bb, &IdiomsCommon::exchangeBitShiftUDiv,

src/bin2llvmir/optimizations/idioms/idioms_common.cpp

@@ -190,29 +190,6 @@ Instruction * IdiomsCommon::exchangeBitShiftSDiv1(BasicBlock::iterator iter) con
 	return res;
 }
 
-/**
- * Exchange shift right with a division.
- *
- * @param iter value to visit
- * @return replaced Instruction, otherwise nullptr
- */
-Instruction * IdiomsCommon::exchangeBitShiftSDiv2(BasicBlock::iterator iter) const {
-	Instruction & val = (*iter);
-	Value * op0 = nullptr;
-	ConstantInt * cnst = nullptr;
-
-	// X s>> C --> X / 2^C
-	if (match(&val, m_AShr(m_Value(op0), m_ConstantInt(cnst))) &&
-			isPowerOfTwoRepresentable(cnst)) {
-		Constant *NewCst = ConstantInt::get(cnst->getType(),
-						pow(2, *cnst->getValue().getRawData()));
-		BinaryOperator *div = BinaryOperator::CreateSDiv(op0, NewCst);
-		return div;
-	}
-
-	return nullptr;
-}
-
 /**
  * Exchange shift left by with a multiplication
  *