R600/SI: Update concat_vectors.ll to check for scratch usage

These tests were using SI-NOT: MOVREL to make sure concat vectors
weren't being lowered to stack loads and stores, but we are using
scratch buffers for the stack now instead of registers, so we need
to add an additional SI-NOT check for scratch buffers.

With this change I was able to uncover one broken test which will
be fixed in a future commit.
R600/SI: Custom lower CONCAT_VECTORS

This will lower them using register copies rather than loads and stores
to the stack.