Added back logic in patch r46361...

Added back logic in patch r46361 (http://llvm.org/viewvc/llvm-project?rev=46361&view=rev) with the addition of
some previously missing NULL pointer checks.

Modified the UninitializedValues analysis to not expect that every Expr* at the
block-level is a block-level expression (we probably need to change the name of
such expressions to something truer to their meaning).

llvm-svn: 46380