[PR16756] Add SSAUpdaterBulk.
SSAUpdater is a bottleneck in a number of passes, and one of the reasons
is that it performs a lot of unnecessary computations (DT/IDF) over and
over again. This patch adds a new SSAUpdaterBulk that uses existing DT
and avoids recomputing IDF when possible.
Reviewers: dberlin, davide, MatzeB
Subscribers: llvm-commits, hiraditya
Differential Revision: https://reviews.llvm.org/D44282