i965: Do our lowering passes before the loop of optimization.

i965: Do our lowering passes before the loop of optimization.

The optimization loop won't reinsert noise instructions or quadop
vectors, so we were traversing the tree for nothing.  Lowering vector
indexing was in the loop after do_common_optimization() to avoid the
work if it ended up that the index was actually constant, but that has
been called already in the core.