PSA: Freeze on new proposals starting Nov 13.

[tlively]

We decided in our sync meeting this morning that any proposals for new instructions submitted on or after November 13 will be automatically labeled "post SIMD MVP." This policy does not apply to modifications of existing proposals, including but not limited to modifications to the particular instructions included in an existing proposal, their names, their precise semantics, or their opcodes.

We also plan to decide on and announce further cutoff dates at our November 13 sync meeting.