[InstCombine] Support (sub (sext x), (sext y)) --> (sext (sub x, y)) and (sub (zext x), (zext y)) --> (zext (sub x, y))
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.
Reviewed By: spatel
Differential Revision: https://reviews.llvm.org/D52075