nir: Add a helper for general instruction-modifying passes.

nir: Add a helper for general instruction-modifying passes.

The nir_shader_lower_instructions() is really nice, but it's only for SSA
operations, and sometimes you want something more general.  I've put it in
nir_builder.h so it can be inlined and retain the same performance
characteristics we're used to in our lowering passes even in the absence
of LTO.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6412>