A signed bitfield's range is [-1,0], so assigning 1 is technically an...

A signed bitfield's range is [-1,0], so assigning 1 is technically an overflow. However, the other bitfield requires a signed value (it supports negative offsets), so it is slightly better to retain a signed 1-bit bitfield and use -1 instead of 1. Silences an MSVC warning.

llvm-svn: 260973