Simplify common predecessor finding.

- Walking over pred_begin/pred_end is an expensive operation.
- PHINodes contain a value for each predecessor anyway.
- While it may look like we used to save a few iterations with the set,
  be aware that getIncomingValueForBlock does a linear search on
  the values of the phi node.
- Another -5% on ARMDisassembler.cpp (Release build). This was the last
  entry in the profile that was obviously wasting time.

llvm-svn: 145937