nir/constant_expressions: Don't switch on bit size when not needed

nir/constant_expressions: Don't switch on bit size when not needed

For opcodes such as the nir_op_pack_64_2x32 for which all sources and
destinations have explicit sizes, the bit_size parameter to the evaluate
function is pointless and *should* do nothing.  Previously, we were
always switching on the bit_size and asserting if it isn't one of the
sizes in the list.  This generates way more code than needed and is a
bit cruel because it doesn't let us have a bit_size of zero on an ALU op
which shouldn't need a bit_size.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>