[ValueTracking] Move GlobalAlias handling to be after the max depth check in computeKnownBits()

GlobalAlias handling used to be after GlobalValue handling, which meant it was, in practice, dead code. r220165 moved GlobalAlias handling to be before GlobalValue handling, but also moved it to be before the max depth check, causing an assert due to a recursion depth limit violation. 

This moves GlobalAlias handling forward to where it's safe, and changes the GlobalValue handling to only look at GlobalObjects.

Differential Revision: http://reviews.llvm.org/D6758

llvm-svn: 224765