Extended Description

First, the trivial thing: a switch lowered to a series of conditional branches always results in an explicit
"fallthrough" uncond branch to the next block from the initial block. This is probably a minor book-
keeping bug:

    cmpl $6, %eax
    jl LBB1_20      #entry

*** jmp LBB1_15 #entry
LBB1_15: #entry

More significantly, we should change the heuristic of the switch lowering a bit. Specifically, right now,
at the top level, we decide whether to lower the switch to a series of branches or to a table jump. I
think we should always go through the "branch sequence" code path, but make it a bit more
sophisticated.

In particular, the worklist should start out with a single record with all the case ranges on it. Each time
through the loop, a range is popped off. If the range satisfies the conditions for a tbljmp, emit one.
Otherwise, split in half and emit a branch to choose between the two.

This would allow us to handle switches with dense portions that are spread out (e.g. 0...100 +
2000...2100) as two separate tbljump pieces.

This would also make it more natural to plug in other heuristics. For example, perlbmk has a switch
that looks like this:
switch uint %tmp158, label %bb336 [
uint 0, label %bb338
uint 1, label %bb338
uint 2, label %bb338
uint 3, label %bb338
uint 4, label %bb338
uint 5, label %bb338
uint 6, label %bb338
uint 7, label %bb
uint 8, label %bb338
uint 9, label %bb322
uint 10, label %bb324
uint 11, label %bb326
uint 12, label %bb328
uint 13, label %bb330
uint 14, label %bb332
uint 15, label %bb334
...

When emitted as a branch tree, the leaves end up doing lots of work to discriminate between the 0->6
cases, when they all go to the same place.

Another nice xform is to do the "shift/and" technique for switches.

-Chris