highbd_idct8x8_add_sse4: make << of neg. val a multiply
highbd_idct8x8_add_sse4: make << of neg. val a multiply

left shifting a negative value is undefined; quiets a ubsan warning.
this is applied to a constant, no change in the generated code.

Change-Id: Ia17a7672d4832463decbc4afd6cd42974d02698e

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx