[GlobalISel] Tidy up unnecessary calls to createGenericVirtualRegister

Summary:
As a side effect some redundant copies of constant values are removed by
CSEMIRBuilder.

Reviewers: aemerson, arsenm, dsanders, aditya_nandakumar

Subscribers: sdardis, jvesely, wdng, nhaehnle, rovka, hiraditya, jrtc27, atanasyan, volkan, Petar.Avramovic, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73789