[ADCE] Use MapVector for BlockInfo to make iteration order deterministic

Summary:
Also added a reserve() method to MapVector since we want to use that from
ADCE.

DenseMap does not provide deterministic iteration order so with that
we will handle the members of BlockInfo in random order, eventually
leading to random order of the blocks in the predecessor lists.

Without this change, I get the same predecessor order in about 90% of the
time when I compile a certain reproducer and in 10% I get a different one.

No idea how to make a proper test case for this.

Reviewers: kuhar, david2050

Reviewed By: kuhar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39593

llvm-svn: 317323