[ValueLattice] Remove obsolete getConstantInt (NFC).

ConstantInt values are always represented as constant ranges with a
single element. getConstantInt is obsolete, as pointed out by @nikic
during D60581.

Reviewers: nikic

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D74329

llvm/include/llvm/Analysis/ValueLattice.h

@@ -281,12 +281,6 @@ public:
     return true;
   }
 
-  ConstantInt *getConstantInt() const {
-    assert(isConstant() && isa<ConstantInt>(getConstant()) &&
-           "No integer constant");
-    return cast<ConstantInt>(getConstant());
-  }
-
   /// Compares this symbolic value with Other using Pred and returns either
   /// true, false or undef constants, or nullptr if the comparison cannot be
   /// evaluated.