[InstCombine] Add test for miscompile in gep-of-gep fold (NFC)

Author: nikic

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -1680,4 +1680,15 @@ if.else:
   ret i64 0
 }
 
+
+@g = external global i8
+
+; FIXME: This is a miscompile
+define ptr @constexpr_gep_of_gep_with_narrow_type() {
+; CHECK-LABEL: @constexpr_gep_of_gep_with_narrow_type(
+; CHECK-NEXT:    ret ptr getelementptr (i8, ptr @g, i64 -2)
+;
+  ret ptr getelementptr (i8, ptr getelementptr (i8, ptr @g, i8 127), i8 127)
+}
+
 !0 = !{!"branch_weights", i32 2, i32 10}