[llvm] [TableGen] Avoid repeated hash lookups (NFC) (PR #120681)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

Your patch nerd-sniped me because with your change, _technically_ `InstResults` will store a different value -- the original destination register rather than the one that comes after -- upon running into collision key, since `I.error(...)` won't bail out from the execution. However, I think even the flow won't stop right away with this ill-formed tree pattern instance, it will eventually top at the type inference stage that follows. 

https://github.com/llvm/llvm-project/pull/120681