Fix get_expr_and_block_pc buglet.

* Clean up and add more comments.

* Fix get_expr_block_and_pc uninitialized $pc buglet.

* Fix unneeded "if" in get_gcc_jit_context.  Possibly wrong fix. For
  some reason this function has been made non lazy, which means we are
  leaking solib reference per compiler run, and opening the solib
  over. The move from global fe_context to local fe_context is
  incomplete, possibly requiring a static variable.

* Remove public export of get_gcc_jit_context.

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