[InstCombine] Support (sub (sext x), (sext y)) --> (sext (sub x, y)) and (sub (zext x), (zext y)) --> (zext (sub x, y))

[craig.topper]

Summary

If the sub doesn't overflow in the original type we can move it above the sext/zext.

This is similar to what we do for add. The overflow checking for sub is currently weaker than add, so the test cases are constructed for what is supported.

[spatel]

Logic seems right, but we already had a big pile of repeated code.
I think we can handle sub the same way as add/mul after:
rL342292
The only extra work will be swapping the constant operand to LHS for sub?

[craig.topper]

Rebase to use the code from r342292.

[spatel]

LGTM - but please add at least one negative test where the math actually could overflow, so we verify that the value tracking is in place.