swift234

Use AddSSAPasses to add low-level passes.

This eliminates a pretty similar list of passes added in a similar order
with just re-using the ordering from AddSSAPasses. Beyond the particular
inliner pass (which is maintained with this change), there was nothing
really specific to low-level code with the order that was present before.

I measure a 1% increase in compile time of the stdlib, no perf
regressions (at -O), and a few decent improvements:
 19 CaptureProp                           5233             4129     -1104    -21.1%     1.27x
 30 ErrorHandling                         3053             2678      -375    -12.3%     1.14x
 65 Sim2DArray                             610              518       -92    -15.1%     1.18x

I expect to be able to get back the 1% compile-time hit (and probably
more) with future changes.