[SCEVExpander] Use stable_sort to sort loop Phis in SCEVExpander::replaceCongruentIVs

This is a fix for test failures on expensive checks build caused by db289340.

With LLVM_ENABLE_EXPENSIVE_CHECKS enabled the llvm::sort shuffles the given container.
However, the sort is only called when the TTI is passed to replaceCongruentIVs.
In the mentioned patch we pass it TTI, so the sort happens. But due to shuffling
equivalent Phis may appear in different order from run to run.
With the stable_sort instead of sort this is impossible - the order of sorted Phis
is preserved.