[Verifier] Avoid visiting DIGlobalVariables twice.

We currently visit them twice.
Once, through `visitMDNode()` -> (the code generated by)
  `../include/llvm/IR/Metadata.def:109` -> `visitDIGlobalVariable()`
Then, through `visitMDNode()` -> `visitDIGlobalVariableExpression()`
  -> `visitDIGlobalVariable()`

This results in verification failures printed twice, e.g.:

  $ ./opt -verify ../../test/DebugInfo/pr34186.ll
  missing global variable type
  !4 = distinct !DIGlobalVariable(name: "pat", scope: !0,
    file: !1, line: 27, isLocal: true, isDefinition: true)
  missing global variable type
  !4 = distinct !DIGlobalVariable(name: "pat", scope: !0,
    file: !1, line: 27, isLocal: true, isDefinition: true)
  ./opt: ../../test/DebugInfo/pr34186.ll: error: input module is broken!

The patch removes one call so we ensure each GV is visited exactly once.

Differential Revision:  https://reviews.llvm.org/D36797

llvm-svn: 311081