microsoft/clc: Use ALU source to determine bitsize lowering
In our lower_bit_size callback, we were previously looking at the destination bitsize to determine whether to convert from 8->16 bits. However, binary operations such as ilt have a 1-bit dest bitsize, so we will not trigger conversions for these ops.
Rewrite the callback to look at the source bitsize instead, which allows us to see through more ops.