vpx_dsp_common.h,clip_pixel: work around VS2022 Arm64 issue

cl.exe targeting AArch64 with optimizations enabled
produces invalid code for clip_pixel() when the return type is uint8_t.
See:
https://developercommunity.visualstudio.com/t/Misoptimization-for-ARM64-in-VS-2022-17/10363361

Bug: b/277255076
Bug: webm:1788
Change-Id: Ia3647698effd34f1cf196cd33fa4a8cab9fa53d6
1 file changed