[AMDGPU] NFC, Comment in disassembler for dpp8

Gives reasoning for convertDPP8.
Also corrects typo in Operand type comment.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D100665

Change-Id: I33ff269db8072d83e5e0ecdbfb731d6000fc26c4
GitOrigin-RevId: 919236e608685e1c5c43edb03b42b4d8acab45aa
2 files changed