Avoid compound assignment with volatile left operand as described in #1017

[jonathangjertsen]

Fixes #1017

Checked by compiling a C++ file that includes all the same files included by kitchen_sink.c.

[kilograham]

As per #1017 this C++ change will be undone, however...

All except the interp one should be using hw_set_bits anyway, so feel free to update this PR to do that, and then leave the interp = interp | as you have it here, as one such occurrence is probably not worth adjusting warnings for