[RISCV] Add DAG combine to detect opportunities to replace (i64 (any_extend...

[RISCV] Add DAG combine to detect opportunities to replace (i64 (any_extend (i32 X)) with sign_extend.

If type legalization is going to insert a sign_extend for other users
of X and we can fold the sign_extend into ADDW/MULW/SUBW, it is
better to replace the ANY_EXTEND so we don't end up with a separate
ADD/MUL/SUB instruction for the users of the ANY_EXTEND.

I'm only handling setcc uses right now, but there are other
instructions that force sign_extends like ashr.

There are probably other *W instructions we could use in addition
to ADDW/SUBW/MULW.

My motivating case was a loop terminating compare and a phi use
as seen in the new test file.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D104581