opt: Update copy prop arrays to handle InterpolateAt* instructions

[cassiebeckley]

Adds support for InterpolateAtCentroid, InterpolateAtSample, and InterpolateAtOffset to the copy propagate arrays pass, as well as propagating pointers with the Input storage class. Also handles situations where variables should be propagated in an order different than the order that they are declared in the source.

[s-perron]

This look good. I might replace the repeated switch statement with a function call

bool IsInterpolationInstruction(Instruction* inst);

[cassiebeckley]

@s-perron the discussed changes have been finished