[llvm] [LLVM][Intrinsics] Change overly defensive code in `DecodeIITType` (PR #190260)

[Rahul Joshi via llvm-commits]

jurahul wrote:

AArch64 failure is an unrelated "Unexpectedly passed" test in Flang.

```
2026-04-11T22:32:26.6360846Z Unexpectedly Passed Tests (1):
2026-04-11T22:32:26.6361169Z   Flang :: Driver/save-mlir-temps.f90
```

https://github.com/llvm/llvm-project/pull/190260