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

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

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.

Reviewers: spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52075

llvm-svn: 342335