[MLIR][Bytecode] Ensure dataIt is aligned coming out of EncodingReader::alignTo.

This addresses the TODO in the code previously and checks that the address of `dataIt` is properly aligned to the requested alignment.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D137855

mlir/lib/Bytecode/Reader/BytecodeReader.cpp

@@ -103,8 +103,13 @@ public:
       }
     }
 
-    // TODO: Check that the current data pointer is actually at the expected
-    // alignment.
+    // Ensure the data iterator is now aligned. This case is unlikely because we
+    // *just* went through the effort to align the data iterator.
+    if (LLVM_UNLIKELY(!llvm::isAddrAligned(llvm::Align(alignment), dataIt))) {
+      return emitError("expected data iterator aligned to ", alignment,
+                       ", but got pointer: '0x" +
+                           llvm::utohexstr((uintptr_t)dataIt) + "'");
+    }
 
     return success();
   }