[AMDGPU] More accurate names for dpp operand types

NFC. Renames the variable in the dpp input operand generators
from DstRC to OldRC, because that is what it actually sets.

Also documents the importance of setting HasModifiers = 0 in the
dpp8 asm string.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D103047

Change-Id: Ice69ae38f644de7f228a75ca47c43e88b1f7d9e1