For example, this occurred in a shader:
reg_idx write_mask
0 xyz
1 xyzw
2 xyzw
3 xyz
The dcl_indexrange instruction covered only xyz, so once merged, searching for
xyzw failed.
It is impossible to declare an input array where elements have different
component counts, but the optimiser can create this case. One way for
this to occur is to dynamically index input values via a local array
containing copies of the input values. The optimiser converts this to
dynamically indexed inputs.