[AMDGPU][ConstantFolding] Fold llvm.amdgcn.cube* intrinsics

Summary:
This folds the following family of intrinsics:
llvm.amdgcn.cubeid (face id)
llvm.amdgcn.cubema (major axis)
llvm.amdgcn.cubesc (S coordinate)
llvm.amdgcn.cubetc (T coordinate)

Reviewers: nhaehnle, arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75187