Remove hard-coded hashes from codegen tests
This removes hard-coded hashes from the codegen and assembly tests. These use FileCheck, which supports eliding part of the pattern being matched, including by capturing it as a pattern parameter for later matching-on. This is much more appropriate than asking contributors to engage with deliberately-opaque identifier schemes.
In order to reduce the likelihood of error, every hash-coded segment I've touched now expects a certain length. This correctly represents these cases, as our hash outputs have a predetermined amount of entropy attached to them.
This is not done for the UI test suite as those are comparatively easy to simply `--bless`, whereas that would be inappropriate for codegen tests. It is also not done for debuginfo tests as those tests do not support such elision in a correct and useful way.
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