isProfitableChain was added based on register number cost. But now, there are some targets like X86/PowerPC taking instruction number as the major cost.
So on these targets, isProfitableChain wrongly eliminates some LSRUse based on register number when collecting chains/LSRUses, these eliminated LSRUse may impact the instruction number a lot in later rate cost phase. Thus we will get a suboptimal loop code sequence.
This patch can make some cpu2017 benchmarks improve on PowerPC.