[CodeGen][IfCvt] Don't re-ifcvt blocks with unanalyzable terminators.

If we couldn't analyze its terminator (i.e., it's an indirectbr, or some
other weirdness), we can't safely re-if-convert a predicated block,
because we can't tell whether the predicated terminator can
fallthrough (it does).

Currently, we would completely ignore the fallthrough successor. In
the added testcase, this means we used to generate:

    ...
  @ %entry:
    cmp   r5, #21
    ittt  ne
  @ %cc1f:
    cmpne r7, #42
  @ %cc2t:
    strne.w       r5, [r8]
    movne pc, r10
  @ %cc1t:
    ...

Whereas the successor of %cc1f was originally %bb1.
With the fix, we get the correct:

    ...
  @ %entry:
    cmp   r5, #21
    itt   eq
  @ %cc1t:
    streq.w       r5, [r11]
    moveq pc, r0
  @ %cc1f:
    cmp   r7, #42
    itt   ne
  @ %cc2t:
    strne.w       r5, [r8]
    movne pc, r10
  @ %bb1:
    ...

rdar://20192768
Differential Revision: http://reviews.llvm.org/D8509

llvm-svn: 232872