[PR16756] Add SSAUpdaterBulk.

Summary:
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

llvm-svn: 329643