[ConstantFolding] Support folding loads from a GlobalAlias

The MSVC ABI requires that we generate an alias for the vtable which
means looking through a GlobalAlias which cannot be overridden improves
our ability to devirtualize.

Found while investigating PR20801.

Patch by Andrew Zhogin!

Differential Revision: http://reviews.llvm.org/D11306

llvm-svn: 242955