[InstSimplify] Don't create a constant to look for a constant. Just see if we have a constant and check its value

[craig.topper]

Summary

The code here was creating a signbit constant to do a pattern match. But creating a constant isn't free due to the uniqueing. So its more efficient to just see if a constant is being used and see if its the constant we're interested in.

[craig.topper]

Now I see we already have an m_SignBit which makes this change much more obvious.

lib/Analysis/InstructionSimplify.cpp

static Value *SimplifyAddInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
   Type *Ty = Op0->getType();
   if (match(Op0, m_Not(m_Specific(Op1))) ||
       match(Op1, m_Not(m_Specific(Op0))))
     return Constant::getAllOnesValue(Ty);
 
   // add nsw/nuw (xor Y, signbit), signbit --> Y
   // The no-wrapping add guarantees that the top bit will be set by the add.
   // Therefore, the xor must be clearing the already set sign bit of Y.
-  Constant *SignBit =
-      ConstantInt::get(Ty, APInt::getSignBit(Ty->getScalarSizeInBits()));
-  if ((isNSW || isNUW) && match(Op1, m_Specific(SignBit)) &&
-      match(Op0, m_Xor(m_Value(Y), m_Specific(SignBit))))
+  const APInt *C0, *C1;
+  if ((isNSW || isNUW) && match(Op1, m_APInt(C0)) &&
+      match(Op0, m_Xor(m_Value(Y), m_APInt(C1))) &&
+      C0->isSignBit() && C1->isSignBit())
     return Y;
 
   /// i1 add -> xor.
   if (MaxRecurse && Op0->getType()->isIntegerTy(1))
     if (Value *V = SimplifyXorInst(Op0, Op1, Q, MaxRecurse-1))
       return V;
 
   // Try some generic simplifications for associative operations.