Followup issue after discussion in #3911, to improve debug codegen for a minor editing cost.

As @AlexGuteniev mentioned in #3911 (comment) , <valarray> is currently extremely careful to extract the size to a local variable to avoid aliasing, which is important for vectorizing its following loops. When replacing calls of size() with direct access to _Mysize, the local variables should be preserved. We may even want to add comments like // avoid aliasing as a small reminder about this somewhat unusual pattern.