Update legalization passes

Update legalization passes

I've a few passes the legalization passes.  The first is to add the
more specialized load-store removal passes to help improve the compile
time, as was suggested in #1118.

I've also added dead branch elimination while we wait for the behaviour
of dead branch elimination to be folded into CFG cleanup.

I did not add CCP because it seems like most of the constant propagation
what is needed is already being done by the load-store removal passes,
which call `ReplaceAllUsesWith`.  We can reconsider this if needed.