Fix BOX import to not create "wrong" casts (dotnet#57094)

All casts in the IR have TYP_INT, except for this one
case. Fix it by using genActualType.

Also add a comment explaining the reason a cast is needed
at all in case the types are small.

src/coreclr/jit/importer.cpp

@@ -6855,18 +6855,26 @@ void Compiler::impImportAndPushBox(CORINFO_RESOLVED_TOKEN* pResolvedToken)
             {
                 lclTyp = JITtype2varType(jitType);
             }
-            assert(genActualType(exprToBox->TypeGet()) == genActualType(lclTyp) ||
-                   varTypeIsFloating(lclTyp) == varTypeIsFloating(exprToBox->TypeGet()));
+
             var_types srcTyp = exprToBox->TypeGet();
             var_types dstTyp = lclTyp;
 
+            // We allow float <-> double mismatches and implicit truncation for small types.
+            assert((genActualType(srcTyp) == genActualType(dstTyp)) ||
+                   (varTypeIsFloating(srcTyp) == varTypeIsFloating(dstTyp)));
+
+            // Note regarding small types.
+            // We are going to store to the box here via an indirection, so the cast added below is
+            // redundant, since the store has an implicit truncation semantic. The reason we still
+            // add this cast is so that the code which deals with GT_BOX optimizations does not have
+            // to account for this implicit truncation (e. g. understand that BOX<byte>(0xFF + 1) is
+            // actually BOX<byte>(0) or deal with signedness mismatch and other GT_CAST complexities).
             if (srcTyp != dstTyp)
             {
-                assert((varTypeIsFloating(srcTyp) && varTypeIsFloating(dstTyp)) ||
-                       (varTypeIsIntegral(srcTyp) && varTypeIsIntegral(dstTyp)));
-                exprToBox = gtNewCastNode(dstTyp, exprToBox, false, dstTyp);
+                exprToBox = gtNewCastNode(genActualType(dstTyp), exprToBox, false, dstTyp);
             }
-            op1 = gtNewAssignNode(gtNewOperNode(GT_IND, lclTyp, op1), exprToBox);
+
+            op1 = gtNewAssignNode(gtNewOperNode(GT_IND, dstTyp, op1), exprToBox);
         }
 
         // Spill eval stack to flush out any pending side effects.