[MachineCSE] Rewrite a loop checking if a block is in a set of blocks without using a set. NFC.

Summary:
Using a set is unnecessary here an in some cases (see e.g. PR37277)
takes significant amount of time to just insert values into it. In this
particular case all we need is just to check if we find the block we are
looking for or not.

Reviewers: davide

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D46411

llvm-svn: 331502