Since JumpLoop is always backwards, and other jumps are always forwards,
we can store the jump offset as an always positive integer and decide on
the jump direction based on the bytecode. This will save a small amount
of space for large-ish for loops (>128 bytecodes).

Review-Url: https://codereview.chromium.org/2641443002
Cr-Commit-Position: refs/heads/master@{#42638}