swift234

[devirtualization] Simplify/refactor optimizeClassMethod.

The only case which we were handling in the old code where the class
method was not dead was when we had an apply inst of the class method.

This commit simplifies all of the weird code therein by causing class
method optimization to go through the same optimization pathway as how
we optimize protocol methods and witness methods (i.e. we pattern match
on the apply and only replace the apply).

This makes the code much simpler and more readable.

Additionally while working on test cases I noticed that relying on
SILCombine to peephole convert_function creates phase ordering issues
since SILCombine does not cause additional optimizer iterations to run
implying that we can have a situation where we devirtualize, fail to
inline, then silcombine (which would allow us to optimize), but then
the pass manager does not go around another time. Thus I move that
operation into the devirtualizer itself since it is relatively simple to
do.

Also re-enable test/SILPasses/devirt_override.sil since we handle it
correctly now.

Swift SVN r17566