instrumentation: Move ExecProcNodeInstr to allow inlining

instrumentation: Move ExecProcNodeInstr to allow inlining

This moves the implementation of ExecProcNodeInstr, the ExecProcNode variant
that gets used when instrumentation is on, to be defined in instrument.c
instead of execProcNode.c, and marks functions it uses as inline.

This allows compilers to generate an optimized implementation, and shows a 4
to 12% reduction in instrumentation overhead for queries that move lots of
rows.

Author: Lukas Fittl <lukas@fittl.com>
Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAP53PkzdBK8VJ1fS4AZ481LgMN8f9mJiC39ZRHqkFUSYq6KWmg@mail.gmail.com