Translation of OpSpecConstantOp to LLVM constant expressions to be implemented for integer and pointer ops

Translation of OpSpecConstantOp to LLVM IR is not yet implemented.

This issue is for implementing translation of the pointer and integer ops to LLVM constant expressions, as required for Kernel SPIR-V.

It is acceptable for the results of pointer operations (including conversions from pointer to integer) not to be constant folded as would be needed for those results to be available at translation time. It will therefore not be possible to use a pointer-related OpSpecConstantOp result to determine the size of an array, for example.

Start by locating the existing OpSpecConstantOp translation path and the handling of LLVM constant expressions. Implement coverage for the specified integer and pointer operations, allowing pointer-operation results to remain unfurled rather than constant-folded; done means the relevant Kernel SPIR-V translations are supported.

Written by the indexing model from the issue text.