Add support for widening integral casts.

Flesh out the SetCC support... which currently ends in a little bit
of unfinished code (which is probably completely hilarious) for
generating the condition value splitting the basic block up into 4
blocks, like this (clearly a better API is needed for this!):

       BB
   cond. branch
     /         /          R1=1    R2=0
     \      /
      \    /
    R=phi(R1,R2)

Other minor edits.

llvm-svn: 13423