Reduce resulting file size by ommiting OpName instructions

I've noticed that the translator generates bunch of `OpName` instructions which actually don't have any semantic impact and can safely be removed from a module. And by doing so, we can reduce size of the generated SPIR-V file.

As an example, I will take a reproducer from #357:

By default, the translator generates 1379560 bytes (~1.347 KBytes) SPIR-V binary.
If I generate input LLVM IR with `-fdiscard-value-names`, resulting SPIR-V binary will only take 1007104 bytes (~983 KBytes).

On this example, by omitting unnecessary `OpName` instructions, we can reduce the resulting binary size by ~27%.

I agree that for text representation `OpName` is still important thing and I guess that removing these instructions will reduce readability of text representation generated by SPIR-V Tools as well, but we can at least add option similar to `-fdiscard-value-names` from clang.

What do you think?

Start by reviewing the reproducer from issue #357 and the translator's handling of OpName instructions. Compare binary output with and without the proposed option, and consider the stated impact on SPIR-V Tools text output. Done means an option can omit unnecessary OpName instructions and the reported binary-size reduction is demonstrated.

Written by the indexing model from the issue text.