[AMDGPU] Split Dpp8FI and Dpp16FI operands (#82379)

Split Dpp8FI and Dpp16FI into two different operands sharing an
AsmOperandClass. They are parsed and rendered identically as fi:1 but
the encoding is different: for DPP16 FI is a single bit, but for DPP8 it
uses two different special values in the src0 field. Having a dedicated
decoder for Dpp8FI allows it to reject other (non-special) src0 values
so that AMDGPUDisassembler::getInstruction no longer needs to call
isValidDPP8 to do post hoc validation of decoded DPP8 instructions.