[AMDGPU] Fix incorrect MachineMemOperand offsets and sizes in wide s_buffer_load splits (#189890)
When G_AMDGPU_S_BUFFER_LOAD (or its SelectionDAG equivalent) falls back
to MUBUF due to a divergent offset, wide loads (256-bit, 512-bit) are
split into multiple 128-bit chunks. Both code paths that perform this
split had bugs in how they annotated MachineMemOperand (MMO) metadata on
each chunk instruction — reporting wrong offsets and wrong sizes. This
does not affect generated assembly correctness but degrades the analysis
if that MMO metadata is used.
---------
Co-authored-by: Abhinav Garg <abhigarg@amd.com>
Co-authored-by: Jay Foad <jay.foad@gmail.com>