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

[bzcheeseman]

Summary

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

[rriddle]

Is there a test we could write for this?

[bzcheeseman]

In D137855#3923083, @rriddle wrote:

Is there a test we could write for this?

Honestly I'm not sure how you'd ever hit this case given that dataIt was just aligned by the previous while loop. I actually debated just making it an assert cause the only time I could think of it might happen is if the address is changed under the hood or something.

mlir/lib/Bytecode/Reader/BytecodeReader.cpp

while (uintptr_t(dataIt) & (uintptr_t(alignment) - 1)) {
       if (failed(parseByte(padding)))
         return failure();
       if (padding != bytecode::kAlignmentByte) {
         return emitError("expected alignment byte (0xCB), but got: '0x" +
                          llvm::utohexstr(padding) + "'");
       }
     }
 
-    // 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();
   }
 
   /// Emit an error using the given arguments.
   template <typename... Args>
   InFlightDiagnostic emitError(Args &&...args) const {
     return ::emitError(fileLoc).append(std::forward<Args>(args)...);