LowerSwitch: Avoid inserting NewDefault block

The NewDefault was used to simplify the updating of PHI nodes, but it
causes some inefficiency for target that will run structurizer later. For
example, for a simple two-case switch, the extra NewDefault is causing
unstructured CFG like:

        O
       / \
      O   O
     / \ / \
    C1  ND C2
     \  |  /
      \ | /
        D

The change is to avoid the ND(NewDefault) block, that is we will get a
structured CFG for above example like:

        O
       / \
      /   \
     O     O
    / \   / \
   C1  \ /  C2
    \-> D <-/

The IR change introduced by this patch should be trivial to other targets,
so I am doing this unconditionally.

Fall-through among the cases will also cause unstructured CFG, but it need
more work and will be addressed in a separate change.

Reviewed by: arsenm

Differential Revision: https://reviews.llvm.org/D123607