Infinite loop fix to constant propagation optimisation

Summary

This merge request fixes an infinite loop that can occur in the CONSTPROP optimisation. This occurs in a piece of code that converts an Inc/Dec statement into Add/Sub to remove side-effects, but firstpass is forcibly run again and this converts it back to an Inc/Dec node, which is then converted into Add/Sub by CONSTPROP, ad infinitum. This tends to only occur if USELOADMODIFYSTORE is disabled.

This patch prevents this by making the Add/Sub node 'internal', and then looking out for that flag when CONSTPROP is run again (since the flag is preserved when firstpass transforms it back) = if the 'internal' flag is present, the Inc/Dec node isn't transformed into Add/Sub, since it's already been done once before, and hence CONSTPROP is not caught in an infinite loop and can move on.

System

Platform-agnostic, but developed and tested on x86_64-win64.

What is the current bug behavior?

When building the compiler with make clean all OPT="-O3 -OoNOUSELOADMODIFYSTORE", the compiler gets caught in an infinite loop while building the System unit with ppc1.

What is the behavior after applying this patch?

The compiler should build successfully with these flags.

Relevant logs and/or screenshots

No logs to post since it just hangs until the process is forcibly terminated.