[AMDGPU] Base getSubRegFromChannel on TableGen data

Generate (at runtime) the table used to drive getSubRegFromChannel,
base on AMDGPUSubRegIdxRanges from TableGen data.
The is a step closer to it being staticly generated by TableGen and
allows getSubRegFromChannel handle all bitwidths in the mean time.

Reviewed By: rampitec, arsenm, foad

Differential Revision: https://reviews.llvm.org/D89217