[AMDGPU][GFX12] Default component broadcast store (#76212)

For image and buffer stores the default behaviour on GFX12 is to set all
unset components to the value of the first component. So if we pass only
X component, it will be the same as XXXX, or XY same as XYXX.

This patch simplifies the passed vector of components in InstCombine by
removing components from the end that are equal to the first component.

For image stores it also trims DMask if necessary.

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Co-authored-by: Mateja Marjanovic <mmarjano@amd.com>