Added initial support into the flow-sensitive dataflow solver to visit the Block-level expression

in a block's terminator.  This expression is visited within a block, but it is accessed by the
terminator.  This is important to observe because for live-variables analysis the block-level
expression is live between the terminator and where the expression occurs in the block.  So far
this hasn't been an issue to not observe this because the block-level expression used in the
terminator is always the last one in the block, and we have never queried the liveness information
about this point (but before the terminator).

llvm-svn: 49709