For PR1258:

Radically simplify the SlotMachine. There is no need to keep Value planes
around any more. This change causes slot numbering to number all un-named,
non-void values starting at 0 and incrementing monotonically through the
function, regardless of type (including BasicBlocks). Getting slot numbers
is now a single lookup operation instead of a double lookup.

llvm-svn: 35171