add a special case for codegen that improves the case where we have

multiple sequential cases to a) not create tons of fall-through basic blocks
and b) not recurse deeply.  This fixes codegen on 100K deep cases, and improves
codegen on moderate cases from this:

        switch i32 %tmp, label %sw.epilog [
                i32 1000, label %sw.bb
                i32 1001, label %sw.bb1
                i32 1002, label %sw.bb2
                i32 1003, label %sw.bb3
                i32 1004, label %sw.bb4
...
sw.bb:          ; preds = %entry
        br label %sw.bb1

sw.bb1:         ; preds = %entry, %sw.bb
        br label %sw.bb2

sw.bb2:         ; preds = %entry, %sw.bb1
        br label %sw.bb3

sw.bb3:         ; preds = %entry, %sw.bb2
        br label %sw.bb4

to:

        switch i32 %tmp, label %sw.epilog [
                i32 1000, label %sw.bb
                i32 1001, label %sw.bb
                i32 1002, label %sw.bb
                i32 1003, label %sw.bb
                i32 1004, label %sw.bb
sw.bb:  ;; many preds

llvm-svn: 66015