Optimize abs operations in C2 compiler

In current SLP, the vectorization of Math.abs() with char type is intentionally disabled after JDK-8261022 because it generates incorrect result before.

But in Java semantics, as the char type is always non-negative, we could actually translate Math.abs(char) to a no-op by adding a no-op IR and matching rules somehow.

We can consider re-enable Math.abs() vectorization with char type.