[PR16756] Use SSAUpdaterBulk in JumpThreading.

Summary:
SSAUpdater is a bottleneck in JumpThreading, and this patch improves the
situation by using SSAUpdaterBulk instead.

Compile time impact: no noticable changes on CTMark, a big improvement
on the test from PR16756.

Reviewers: dberlin, davide, MatzeB

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D44282

llvm-svn: 329644