Port LowerVectorTransfers from EDSC + AST to declarative builders

This CL removes the dependency of LowerVectorTransfers on the AST version of EDSCs which will be retired.

This exhibited a pretty fundamental staging difference in AST-based vs declarative based emission.

Since the delayed creation with an AST was staged, the loop order came into existence after the clipping expressions were computed.
This now changes as the loops first need to be created declaratively in fixed order and then the clipping expressions are created.
Also, due to lack of staging, coalescing cannot be done on the fly anymore and
needs to be done either as a pre-pass (current implementation) or as a local transformation on the generated IR (future work).

Tests are updated accordingly.

PiperOrigin-RevId: 238971631