Make block placement deterministic

We fail to produce bit-to-bit matching stage2 and stage3 compiler in PGO
bootstrap build. The reason is because LoopBlockSet is of SmallPtrSet type
whose iterating order depends on the pointer value.

This patch fixes this issue by changing to use SmallSetVector.

Differential Revision: http://reviews.llvm.org/D26634

llvm-svn: 287148