StructurizeCFG: Set Undef for non-predecessors in setPhiValues()

During structurization process, we may place non-predecessor blocks
between the predecessors of a block in the structurized CFG. Take
the typical while-break case as an example:
```
 /---A(v=...)
 |  / \
 ^ B   C
 |  \ /|
 \---L |
     \ /
      E (r = phi (v:C)...)
```
After structurization, the CFG would be look like:
```
 /---A
 |   |\
 |   | C
 |   |/
 |   F1
 ^   |\
 |   | B
 |   |/
 |   F2
 |   |\
 |   | L
 \   |/
  \--F3
     |
     E
```
We can see that block B is placed between the predecessors(C/L) of E.
During phi reconstruction, to achieve the same sematics as before, we
are reconstructing the PHIs as:
  F1: v1 = phi (v:C), (undef:A)
  F3: r = phi (v1:F2), ...
But this is also saying that `v1` would be live through B, which is not
quite necessary. The idea in the change is to say the incoming value
from B is Undef for the PHI in E. With this change, the reconstructed
PHI would be:
  F1: v1 = phi (v:C), (undef:A)
  F2: v2 = phi (v1:F1), (undef:B)
  F3: r  = phi (v2:F2), ...

Reviewed by: sameerds

Differential Revision: https://reviews.llvm.org/D132450