The ornamental design for a weightlifting supportshirt, as shown and described.
FIG. 1 is a front perspective view of a weightlifting support shirt showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof, the right side view is a mirror image of the left side view;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
FIGS. 1-4 and 6-7, the broken lines depicting lines of stitching represent portions of the weightlifting support shirt that do not form part of the claimed design.