Fixes for Flambda 2 when Config.stack_allocation is false

[mshinwell]

Similarly to #812 , when Config.stack_allocation is false, nothing is to be allocated on the local stack:

1. Don't emit Cmm code for Begin_region and End_region Flambda 2 primitives. These primitives are still present in the Flambda 2 terms because, as of Delete unused local allocation regions in Flambda 2 #809 , region variables are mandatory in allocating primitives and application expressions.
2. Count Begin_region and End_region as having zero code size.

[mshinwell]

This is a bit tricky actually, I'm going to put this directly in #809