Add a standard if op

This CL adds an "std.if" op to represent an if-then-else construct whose condition is an arbitrary value of type i1.
This is necessary to lower all the existing examples from affine and linalg to std.for + std.if.

This CL introduces the op and adds the relevant positive and negative unit test. Lowering will be done in a separate followup CL.

PiperOrigin-RevId: 256649138