[opsem]properly evaluate global pointer in constant struct;

note: directly evaluating a global in a constant struct like this:
%struct.s = type { i16*, i32 } ; struct with a ptr to i16 and a i32 int
@g.ptr =  internal unnamed_addr global i16 42, align 4
%v2.p = extractvalue %struct.s {i16* @g.ptr, i32 0}, 0
will not work due to a memory analysis issue in seadsa: seahorn/sea-dsa#49

test/opsem/insert.extract.value.06.ll

@@ -0,0 +1,49 @@
+; RUN: %seabmc "%s" 2>&1 | %oc %s
+; RUN: %seabmc --horn-bv2-lambdas --log=opsem3 "%s" 2>&1 | %oc %s
+;; testing extracting form const struct with global pointer type fields
+; CHECK: ^unsat$
+; ModuleID = 'test-insert-value'
+source_filename = "/tmp/st.c"
+target datalayout = "e-m:o-p:32:32-f64:32:64-f80:128-n8:16:32-S128"
+target triple = "i386-apple-macosx10.13.0"
+
+%struct.s = type { i16*, i32 } ; struct with a ptr to i16 and a i32 int
+@g.ptr =  internal unnamed_addr global i16 42, align 4
+declare void @verifier.assume.not(i1)
+declare void @seahorn.fail()
+
+
+; Function Attrs: noreturn
+declare void @verifier.error() #0
+
+; Function Attrs: noinline nounwind ssp uwtable
+define i32 @main() local_unnamed_addr #2 {
+entry:
+  ;; %v1 = load i16, i16* @g.ptr
+  %st = insertvalue %struct.s undef, i16* @g.ptr, 0
+  %v2.p = extractvalue %struct.s %st, 0
+  %v2 = load i16, i16* %v2.p
+  %cond = icmp eq i16 42, %v2
+  call void @verifier.assume.not(i1 %cond)
+  br label %verifier.error
+
+verifier.error:                                   ; preds = %entry
+  call void @seahorn.fail()
+  br label %verifier.error.split
+
+verifier.error.split:                             ; preds = %verifier.error
+  ret i32 42
+}
+
+attributes #0 = { noreturn }
+attributes #1 = { argmemonly nounwind }
+attributes #2 = { noinline nounwind ssp uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="penryn" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+sse4.1,+ssse3,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #3 = { nounwind }
+
+!llvm.module.flags = !{!0, !1, !2}
+!llvm.ident = !{!3}
+
+!0 = !{i32 1, !"NumRegisterParameters", i32 0}
+!1 = !{i32 1, !"wchar_size", i32 4}
+!2 = !{i32 7, !"PIC Level", i32 2}
+!3 = !{!"clang version 5.0.2 (tags/RELEASE_502/final)"}