[enco] Generate loops correctly (#1409)

[enco] Generate loops correctly (#1409)

The current implementation of LoopGen assumes that Shuffle is fully
specified over the destination bag.

While previous commit addresses one specific issue, it turns out that
LoopGen still suffers from various asserts during execution, and
generates incorrect code.

This commit rewrites LoopGen to depend on TransferSequence instead of
directly using Shuffle instruction to address all of remaining issues.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>