This optimizes a single index extract whose composite value terminates with a
CompositeConstruct (or ConstantComposite) by evaluating to the correct
component. This was needed for opaque legalization.
This highlights the need/opportunity to improve this optimization to deal
with more complex composite expressions including currently handled ops
plus Null ops and special vector composition. A TODO has been added.
Includes code to deal correctly with OpFunctionParameter. This
is needed by opaque propagation which may not exhaustively inline
entry point functions.
Adds ProcessEntryPointCallTree: a method to do work on the
functions in the entry point call trees in a deterministic order.
Currently only SPV_KHR_variable_pointers is disallowed in passes which
do pointer analysis. Positive and negative tests of the general extensions
mechanism were added to aggressive_dce but cover all passes.
