[AMDGPU] gfx11 Decode wider instructions. NFC

Refactor to pass a templatized size parameter to the decoder to allow wider than
64bit decodes in a later patch.

Contributors:
Jay Foad <jay.foad@amd.com>

Depends on D125261

Patch 5/N for upstreaming of AMDGPU gfx11 architecture.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D125316