Added support to dataflow solver to (when requested) also record dataflow

values for the block-level expressions.

Modified 'LiveVariables' to provide the option to clients to record
liveness information for block-level expressions (using the above feature).

Modified 'DeadStores' to conform to the new interface of 'LiveVariables'.

Modified 'GRConstants' to compute liveness information for block-level
expressions.

llvm-svn: 46137