[AMDGPU] New helper functions to get a register class of a given width

Summary:
Introduce new helper functions getVGPRClassForBitWidth,
getAGPRClassForBitWidth, getSGPRClassForBitWidth and use them to
refactor various other functions that all contained their own lists of
valid register class widths. NFC.

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78311