interp: make getelementptr offsets signed

getelementptr offsets are signed, not unsigned. Yet they were used as
unsigned integers in interp.
Somehow this worked most of the time, until finally there was some code
that did a getelementptr with a negative index.

interp/interpreter.go

@@ -644,11 +644,11 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 		case llvm.GetElementPtr:
 			// GetElementPtr does pointer arithmetic, changing the offset of the
 			// pointer into the underlying object.
-			var offset uint64
+			var offset int64
 			for i := 1; i < len(operands); i += 2 {
-				index := operands[i].Uint()
-				elementSize := operands[i+1].Uint()
-				if int64(elementSize) < 0 {
+				index := operands[i].Int()
+				elementSize := operands[i+1].Int()
+				if elementSize < 0 {
 					// This is a struct field.
 					offset += index
 				} else {
@@ -662,7 +662,7 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 					return nil, mem, r.errorAt(inst, err)
 				}
 				// GEP on fixed pointer value (for example, memory-mapped I/O).
-				ptrValue := operands[0].Uint() + offset
+				ptrValue := operands[0].Uint() + uint64(offset)
 				locals[inst.localIndex] = makeLiteralInt(ptrValue, int(operands[0].len(r)*8))
 				continue
 			}