[AMDGPU] Simplify getNumFlatOffsetBits. NFC.

Previously we considered this field to be either N-bit unsigned or
N+1-bit signed, depending on the instruction. I think it's conceptually
simpler to say that the field is always N+1-bit signed, but some
instructions do not allow negative values.

Differential Revision: https://reviews.llvm.org/D140883