[Coverity] Potential `nullptr` dereference

See details at https://scan.coverity.com/projects/intel-llvm?tab=overview as CID `490176`:

https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/590068b12402262dd9bd0aa1386f2e89babfa8a1/lib/SPIRV/SPIRVReader.cpp#L1109

If we don't expect the cast to fail, then we should use `cast` instead of `dyn_cast`. The former contains an assertion instead of runtime check

Start at lib/SPIRV/SPIRVReader.cpp:1109 and review Coverity CID 490176 for the reported potential nullptr dereference. Determine whether the cast is expected to succeed, then inspect nearby tests or translation checks. Done means the reported warning is addressed with behavior consistent with the project’s expectation and relevant checks pass.

Written by the indexing model from the issue text.