This commits adds handling for spv.ptrcast result being used in a store
instruction, modifying the store to operate on the source type.

The generated OpVectorShuffle was wrong, as the indices we pass are not
to select the vector to sample from, but the position in the vector.

OpenCL is allowed to cast pointers, meaning they can resolve some type
mismatches this way. In logical SPIR-V, those are restricted. This new
pass legalizes such pointer cast when targeting logical SPIR-V.

For now, this pass supports 3 cases we witnessed:
 - loading a vec3 from a vec4*.
 - loading a scalar from a vec*.
 - loading the 1st element of an array.

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Co-authored-by: Steven Perron <stevenperron@google.com>