[Inliner] Modify the merging of min-legal-vector-width attribute to better...

[Inliner] Modify the merging of min-legal-vector-width attribute to better handle when the caller or callee don't have the attribute.

Lack of an attribute means that the function hasn't been checked for what vector width it requires. So if the caller or the callee doesn't have the attribute we should make sure the combined function after inlining does not have the attribute.

If the caller already doesn't have the attribute we can just avoid adding it. Otherwise if the callee doesn't have the attribute just remove the caller's attribute.

llvm-svn: 347841