[compiler][amd64] Attempt at faster PC updates.
Always just assign PC to the current address instead of adding the
difference. This is somewhat faster.
We still do too many updates, though.
As an example it is commont to have 2-3 update_pc in a row.
[compiler][amd64] Added instr_prologue in a lot of places.
Also, load sp register more consistently, and only when