This makes emit_(un)spill even more stupid by removing the logic that
decides what execution size each scratch read or write send message
should have and instead relying on the caller to specify an
appropriate execution size via the builder argument.  This makes sense
because the caller will need to act differently based on the scratch
message width (e.g. emit an additional unspill before the instruction
if the execution width and channel layout of the spill doesn't match
the instruction's).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>