[X86] Teach combineTruncatedArithmetic to push truncate through subtracts...

[X86] Teach combineTruncatedArithmetic to push truncate through subtracts where only one of the inputs is free to truncate.

Fix combineSubToSubus to handle the new DAG to avoid a regression.

There are still regressions in test14/test15/test16. Where it
looks like were trying to set up cases we could match to
umin+trunc+subus but the handling was never finished. The
regression here isn't unique to sub. Its a lost opportunity for
taking an AND with two truncated inputs and producing a larger
AND with a single truncate. The same thing could happen with
any other node we handle in combineTruncatedArithmetic since we
are moving the truncate up the DAG.

Differential Revision: https://reviews.llvm.org/D80483