The ornamental design for a prefillvalve, as shown and described.
FIG. 1 is a left view of a prefill valve showing my new design;
FIG. 2 is a right view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a perspective view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the prefill valve and form no part of the claimed design.