Fix verifier for GlobalAliases to avoid recursing into global initializers.

Fix verifier for GlobalAliases to avoid recursing into global initializers.

The verifier follows GlobalAlias operands so that it can detect cycles of
alias definitions. It was doing this in a way that caused it to also recurse
through initializers for the GlobalValue aliasees, and it would fail when
an initializer refers to a global that is a declaration and not a definition.
This patch causes it to stop recursing when it hits a global definition.
<rdar://problem/17277451>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210734 91177308-0d34-0410-b5e6-96231b3b80d8