One thing to consider...

Just because the compiler could be in the kernel, doesn't mean it has to be.

One could envision a hotplug/modprobe like helper program that the
kernel executes
when it wants to translate from one encoding (say p4) to another (say [e]bpf).

This keeps complexity (compiler) out of the kernel, while still
allowing us to have
the illusion of only one interface to sw/hw.  And it has the nice
benefit of allowing us
to use existing compiler toolchains...