[AMDGPU] Accept arbitrary sized sources in CalculateByteProvider

This allows working with e.g. v8i8 / v16i8 sources.

It is generally useful, but is primarily beneficial when allowing e.g. v8i8s to be passed to branches directly through registers. As such, this is the first in a series of patches to enable that work. However, it effects https://reviews.llvm.org/D155995, so it has been implemented on top of that.

Differential Revision: https://reviews.llvm.org/D159036

Change-Id: Idfcb57dacd0c32cab040fe4dd4ac2ec762750664