Handle dead defs in the if converter.

We had code such as this:
  r2 = ...
  t2Bcc

label1:
  ldr ... r2

label2;
  return r2<dead, def>

The if converter was transforming this to
   r2<def> = ...
   return [pred] r2<dead,def>
   ldr <r2, kill>
   return

which fails the machine verifier because the ldr now reads from a dead def.

The fix here detects dead defs in stepForward and passes them back to the caller in the clobbers list.  The caller then clears the dead flag from the def is the value is live.

llvm-svn: 236660