[GlobalISel][AMDGPU] add legalization for G_FREEZE

Summary:
Copy the legalization rules from SelectionDAG:
-widenScalar using anyext
-narrowScalar using intermediate merges
-scalarize/fewerElements using unmerge
-moreElements using G_IMPLICIT_DEF and insert

Add G_FREEZE legalization actions to AMDGPULegalizerInfo.
Use the same legalization actions as G_IMPLICIT_DEF.

Depends on D77795.

Reviewers: dsanders, arsenm, aqjune, aditya_nandakumar, t.p.northover, lebedev.ri, paquette, aemerson

Reviewed By: arsenm

Subscribers: kzhuravl, yaxunl, dstuttard, tpr, t-tye, jvesely, nhaehnle, kerbowa, wdng, rovka, hiraditya, volkan, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78092