`<valarray>`: Consider replacing `size()` calls with `_Mysize`

[StephanTLavavej]

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

As Alex Guteniev (@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.

[AlexGuteniev]

Fun facts:

• Capturing size() was suggested by Michael Schellenberger Costa (@miscco) for a reason unrelated to vectorization. Back then I had less experience with vectorization and didn't see it also helps vectorizing Valarray loops expansion #1165 (comment)
• There's still a place where we don't capture size - it is _Tidy_deallocate(). Capturing _Mysize there wouldn't normally help vectorization, as the types that are vectorized are normally trivially destructible. For debug codegen if constexpr skipping that part may help

STL/stl/inc/valarray

Lines 558 to 560 in 0fe653a

	for (size_t _Idx = 0; _Idx < _Mysize; ++_Idx) {
	_Destroy_in_place(_Myptr[_Idx]);
	}