ir3: run opt_if after opt_vectorize

nir_opt_vectorize could replace swizzled movs with vectorized movs in a
different block. If this happens with swizzled movs in a then block, it
could leave this block empty. ir3 assumes only the else block can be
empty (e.g., when lowering predicates) so make sure ifs are in that
canonical form again.

This fixes empty predication blocks in some shaders, for example:

predt
predf
...
prede

Signed-off-by: Job Noorman <jnoorman@igalia.com>
Part-of: <mesa/mesa!34272>