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/testdata/basic.ll

@@ -10,6 +10,8 @@ target triple = "x86_64--linux"
 @main.exposedValue1 = global i16 0
 @main.exposedValue2 = global i16 0
 @main.insertedValue = global {i8, i32, {float, {i64, i16}}} zeroinitializer
+@main.gepArray = global [8 x i8] zeroinitializer
+@main.negativeGEP = global ptr null
 
 declare void @runtime.printint64(i64) unnamed_addr
 
@@ -88,6 +90,11 @@ entry:
   %agg2 = insertvalue {i8, i32, {float, {i64, i16}}} %agg, i64 5, 2, 1, 0
   store {i8, i32, {float, {i64, i16}}} %agg2, ptr @main.insertedValue
 
+  ; negative GEP instruction
+  %ngep1 = getelementptr [8 x i8], ptr @main.negativeGEP, i32 0, i32 5
+  %ngep2 = getelementptr [8 x i8], ptr %ngep1, i32 0, i32 -3
+  store ptr %ngep2, ptr @main.negativeGEP
+
   ret void
 }