nir: Add nir_lower_alu_to_scalar.

nir: Add nir_lower_alu_to_scalar.

This is the equivalent of brw_fs_channel_expressions.cpp, which I wanted
for vc4.

v2: Use the nir_src_for_ssa() helper, and another instance of
    nir_alu_src_copy().
v3: Drop the non-SSA support.  All intended callers will have SSA-only ALU
    ops.
v4: Use insert_before, drop stale bcsel/fcsel comment, drop now-unused
    unsupported() function, drop lower_context struct.
v5: Completely rename the pass to nir_lower_alu_to_scalar(), add an assert
    about weird input_sizes[].

Reviewed-by: Jason Ekstrand <jason.ekstrand@iastate.edu>