`rotation_kind` in the `PPRotationOp` operation is missing a constraint on its legal values

[mehrdad2m]

The rotation_kind in the PPRotationOp is defined as a I16Attr but there is no limitation on the values. Since only 2, 4, and 8 are the only relevant values, We should add a verifier or a constraint (preferred) so any invalid IR raises an error.
Currently, merge rotation pass is already implicitly assuming these values to be powers of 2, so if someone tries a goofy value, merge rotation would incorrectly change it to a valid IR and continue without raising an error. e.g.

%0 = qec.ppr ["Y"](9) %q1: !quantum.bit
%1 = qec.ppr ["Y"](9) %0: !quantum.bit

would turn into

%0 = qec.ppr ["Y"](4) %arg0 : !quantum.bit 

After merge-rotation pass.