interp: support integer icmp of ptrtoint

This kind of code might be generated by the switch implementation of
func values. The func value is represented as a ptrtoint, and before
calling it, it is compared against 0.

interp/testdata/consteval.ll

@@ -2,6 +2,7 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64--linux"
 
 @intToPtrResult = global i8 0
+@ptrToIntResult = global i8 0
 
 define void @runtime.initAll() {
   call void @main.init()
@@ -10,6 +11,7 @@ define void @runtime.initAll() {
 
 define internal void @main.init() {
   call void @testIntToPtr()
+  call void @testPtrToInt()
   ret void
 }
 
@@ -25,3 +27,16 @@ b:
   store i8 2, i8* @intToPtrResult
   ret void
 }
+
+define internal void @testPtrToInt() {
+  %zero = icmp eq i64 ptrtoint (i8* @ptrToIntResult to i64), 0
+  br i1 %zero, label %a, label %b
+a:
+  ; should not be reached
+  store i8 1, i8* @ptrToIntResult
+  ret void
+b:
+  ; should be reached
+  store i8 2, i8* @ptrToIntResult
+  ret void
+}