[AMDGPU] gfx11 BUF Instructions

Includes MachineCode layer support and tests, and MIR tests not requiring
CodeGen pass changes.
Includes a small change in SMInstructions.td to correct encoded bits.

Contributors:
Petar Avramovic <Petar.Avramovic@amd.com>
Dmitry Preobrazhensky <dmitry.preobrazhensky@amd.com>

Depends on D125316

Patch 6/N for upstreaming of AMDGPU gfx11 architecture.

Reviewed By: dp, Petar.Avramovic

Differential Revision: https://reviews.llvm.org/D125319