At present, we use `ifdef`s to manually determine when a map definition
should be emitted into the object file. When the object file gets loaded by
the agent, every map therein is created and populated before programs can
be loaded into the kernel.

Given the quest for clang-freedom, we want to pre-compile all programs
and only create and populate maps for features that are enabled at runtime
based on global variables for feature flags.

This commit introduces this capability. It performs static analysis on the BPF
program, splits it into basic blocks and finds global data accesses used in
branching instructions. Then, it can predict whether a branch is always or
never taken.

By occluding unreachable parts of the program, it predicts which maps will
remain in use after the verifier performed its own dead code elimination as
part of the verification process. Before loading, unused maps are removed from
the CollectionSpec, and all of their references are poisoned with simple
immediate load instructions.

This means all maps can now be unconditionally defined, normal branches (if
statements!) can be used, and developers no longer have to worry about when
and which maps are or are not loaded under which conditions. Users don't
have to pay for features they don't use.

Signed-off-by: Dylan Reimerink <dylan.reimerink@isovalent.com>
Signed-off-by: Timo Beckers <timo@isovalent.com>
Co-authored-by: Timo Beckers <timo@isovalent.com>

Signed-off-by: Dylan Reimerink <dylan.reimerink@isovalent.com>