nir: Make vec-to-movs handle src/dest aliasing.

nir: Make vec-to-movs handle src/dest aliasing.

It now emits vector MOVs instead of a series of individual MOVs, which
should be useful to any vector backends.  This pushes the problem of
src/dest aliasing of channels on a scalar chip to the backend, but if
there are any vector operations in your shader then you needed to be
handling this already.

Fixes fs-swap-problem with my scalarizing patches.

v2: Rename to insert_mov(), and add a comment about what it does.
v3: Rewrite the comment.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com> (v3)