[llvm][CycleInfo] Quick look-up for block in cycle.

Use a SetVector to store blocks in a cycle to ensure a quick loop-up when
querying whether the cycle contains a given block. This is along the same lines
as the SmallPtrSet in LoopBase, introduced by commit
be640b28.

To make this work, we also enhance SetVector to support vector operations with
pointers and set operations with const pointers in the same container.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D146136