common: add more bytecode helpers

common: add more bytecode helpers

The common/bytecode/bytecode.c file already contains some helper
functions that are used build bytecode. A following patch will require
helpers for a few more operations. This patch factors them out of the
filter-visitor-generate-bytecode.c file.

No functional changes intended.

Change-Id: I013e0dad3e0264d062e3e8733c05f7fbb4bda70c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479