Greatly simplify the ConvertToCFG pass, converting it from a module pass to a

function pass, and eliminating the need to copy over code and do
interprocedural updates.  While here, also improve it to make fewer empty
blocks, and rename it to "LowerIfAndFor" since that is what it does.  This is
a net reduction of ~170 lines of code.

As drive-bys, change the splitBlock method to *not* insert an unconditional
branch, since that behavior is annoying for all clients.  Also improve the
AsmPrinter to not crash when a block is referenced that isn't linked into a
function.

PiperOrigin-RevId: 227308856