[ConstantFolding] Fix handling of index width != pointer width

Per LangRef:

> The offsets are then added to the low bits of the base address up to the index type width, with silently-wrapping two’s complement arithmetic. If the pointer size is larger than the index size, this means that the bits outside the index type width will not be affected.

Author: nikic

llvm/lib/Analysis/ConstantFolding.cpp

@@ -943,18 +943,21 @@ Constant *SymbolicallyEvaluateGEP(const GEPOperator *GEP,
 
   // If the base value for this address is a literal integer value, fold the
   // getelementptr to the resulting integer value casted to the pointer type.
-  APInt BasePtr(BitWidth, 0);
+  APInt BasePtr(DL.getPointerTypeSizeInBits(Ptr->getType()), 0);
   if (auto *CE = dyn_cast<ConstantExpr>(Ptr)) {
     if (CE->getOpcode() == Instruction::IntToPtr) {
       if (auto *Base = dyn_cast<ConstantInt>(CE->getOperand(0)))
-        BasePtr = Base->getValue().zextOrTrunc(BitWidth);
+        BasePtr = Base->getValue().zextOrTrunc(BasePtr.getBitWidth());
     }
   }
 
   auto *PTy = cast<PointerType>(Ptr->getType());
   if ((Ptr->isNullValue() || BasePtr != 0) &&
       !DL.isNonIntegralPointerType(PTy)) {
-    Constant *C = ConstantInt::get(Ptr->getContext(), Offset + BasePtr);
+    // If the index size is smaller than the pointer size, add to the low
+    // bits only.
+    BasePtr.insertBits(BasePtr.trunc(BitWidth) + Offset, 0);
+    Constant *C = ConstantInt::get(Ptr->getContext(), BasePtr);
     return ConstantExpr::getIntToPtr(C, ResTy);
   }
 

llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll

@@ -6,7 +6,7 @@ target datalayout = "p:16:16:16:8"
 ; The GEP should only modify the low 8 bits of the pointer.
 define ptr @test() {
 ; CHECK-LABEL: define ptr @test() {
-; CHECK-NEXT:    ret ptr null
+; CHECK-NEXT:    ret ptr inttoptr (i16 -256 to ptr)
 ;
   %base = inttoptr i16 -1 to ptr
   %gep = getelementptr i8, ptr %base, i8 1