[sledge] fix LLVM assertion failure in xlate_global
[sledge] fix LLVM assertion failure in xlate_global

Summary:
LLVM.global_initializer casues a cast assertion failure if the value
passed to it is not a GlobalVariable. So we first check if it is a
GlobalVariable and only then ask for an initiliazer.

This is hidden if LLVM is built without assertions.

Reviewed By: jberdine

Differential Revision: D15601632

fbshipit-source-id: e9db23a12

facebook/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer