state_tracker: Fix assertion failures in conditional block movs.

state_tracker: Fix assertion failures in conditional block movs.

If you had a conditional assignment of an array or struct (say, from the
if-lowering pass), we'd try doing swizzle_for_size() on the aggregate
type, and it would assertion fail due to vector_elements==0.  Instead,
extend emit_block_mov() to handle emitting the conditional operations,
which also means we'll have appropriate writemasks/swizzles on the CMPs
within a struct containing various-sized members.

Fixes 20 testcases in es3conform on vc4.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>