This pass identifies arithmetic operators that share an operand and whose
results are used in mutually exclusive cases controlled by a multiplexer, and
merges them together by multiplexing the other operands
The idea behind this simplification is that a N-bit signal X being
compared with an M-bit constant where M>N and the constant has Nth
or higher bit set, it either always succeeds or always fails.
However, the existing implementation only worked with one-hot signals
for some reason. It also printed incorrect messages.
This commit adjusts the simplification to have as much power as
possible, and fixes other bugs.
Before this commit, only unsigned comparisons with all-0 would be
simplified. This commit also makes the code handling such comparisons
to be more rigorous and not abort on unexpected input.
Bogdan Vukobratovic
Clifford Wolf
Keith Rothman
whitequark