[AMDGPU] Introduce never uniform bit field in tablegen

IsNeverUniform can be set to 1 to mark instructions which are
inherently never-uniform/divergent. Enabling this bit in
Writelane instruction for now. To be extended to all required
instructions.

Reviewed By: arsenm, sameerds, #amdgpu

Differential Revision: https://reviews.llvm.org/D143154