compiler: emit NaturallyAligned when element type & index type are the same width

This commit fixes a subtle mistake in alignmentFromTypes that used to
generate Unaligned when element type & index type are the same width.
Fixes #24930.

compiler/GHC/StgToCmm/Prim.hs

@@ -2302,8 +2302,8 @@ alignmentFromTypes :: CmmType  -- ^ element type
                    -> CmmType  -- ^ index type
                    -> AlignmentSpec
 alignmentFromTypes ty idx_ty
-  | typeWidth ty < typeWidth idx_ty = NaturallyAligned
-  | otherwise                       = Unaligned
+  | typeWidth ty <= typeWidth idx_ty = NaturallyAligned
+  | otherwise                        = Unaligned
 
 doIndexOffAddrOp :: Maybe MachOp
                  -> CmmType