GlobalISel: Remove redundant pass initialization

Summary:
All the GlobalISel passes are initialized when the target calls
initializeGlobalISel(), so we don't need to call the initializers
from the pass constructors.

Reviewers: qcolombet, t.p.northover, paquette, dsanders, aemerson, aditya_nandakumar

Reviewed By: aemerson

Subscribers: rovka, kristof.beyls, hiraditya, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63235

llvm-svn: 363642