[NewPM] Add SimpleLoopUnswitchPass to PassRegistry.def

Summary:
Seems to just be missing from PassRegistry.def.

Makes the number of check-llvm failures under new PM go from 2619 to 2581.

Reviewers: hans, ychen, asbirlea, leonardchan

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82422