A small, flag-gated addition to the llvm-mos
backend that makes the compiler emit native 16-bit accumulator code for the
WDC 65816 — int16/uint16 add/sub/and/or/xor become REP #$20 … SEP #$20
regions instead of byte-at-a-time sequences.
The feature (+native-16bit-accumulator) is off by default and no CPU
implies it. The existing C ABI, pointer model, 8-bit X/Y, banking, and the 8-bit
accumulator at function boundaries are all unchanged, so feature-enabled and