[NFC][SafepointIRVerifier] Add alias for set of available values

Introduces usage of AvailableValueSet alias name instead of
DenseSet<const Value *> for better reading.

Patch Author: Daniil Suchkov
Reviewers: mkazantsev, anna, apilipenko
Reviewed By: anna
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D41002

llvm-svn: 320465