spirv-diff should test that merely permuting ids never produces a diff

[jimblandy]

If two SPIR-V programs have identical instructions streams except for some reversible substitution of ids, it seems like it should be possible for spirv-diff to always recognize them as identical. One reason #5218 wasn't caught before is that many of the test programs have %2 = OpTypeVoid, so comparing src and dst ids without applying the mapping just happened to produce matches anyway.

It would be pretty easy to catch mistakes like this by taking every .spvasm file in test/diff/diff_files, permuting its ids, and checking that Differ reports no changes.