[AMDGPU] Trim zero components from buffer and image stores

For image and buffer stores the default behaviour on GFX11 and
older is to set all unset components to zero. So if we pass
only X component it will be the same as X000, or XY same as XY00.

This patch simplifies the passed vector of components in InstCombine
by removing zero components from the end.

For image stores it also trims DMask if necessary.

Reviewed by: arsenm, foad, nhaehnle, piotr