Stack spiller tried to save a temp and reuses spilled LHS to store the results of the whole expression.
However, the LHS is not always spilled into a temp. Readonly fields, for example, stay as they are.

As a result the optimization leads to modification of the readonly field which is not even verifiable.,

The change simply introduces a temp, which, if happens to be temp copy of a temp and thus unnecessary, is elided by the stack optimizer who is in a better position to reason about redundancy of locals.

Fixes #4628