[Coverity] Potential `nullptr` dereference

[AlexeySachkov]

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

SPIRV-LLVM-Translator/lib/SPIRV/SPIRVReader.cpp

Line 1109 in 590068b

Src = ConstantExpr::getPointerCast(dyn_cast<Constant>(Src), IntTy);

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