[MachineLICM] Let targets decide if copy-like instructions are cheap

When checking whether it is profitable to hoist an instruction, the pass
may override a target's ruling because it assumes that all COPY
instructions are cheap, and that may not be the case for all
micro-architectures.

On AArch64 there's 0% difference in performance in LLVM's test-suite.

Author: guy-david

llvm/lib/CodeGen/MachineLICM.cpp

@@ -1219,7 +1219,7 @@ bool MachineLICMImpl::HasHighOperandLatency(MachineInstr &MI, unsigned DefIdx,
 /// Return true if the instruction is marked "cheap" or the operand latency
 /// between its def and a use is one or less.
 bool MachineLICMImpl::IsCheapInstruction(MachineInstr &MI) const {
-  if (TII->isAsCheapAsAMove(MI) || MI.isCopyLike())
+  if (TII->isAsCheapAsAMove(MI) || MI.isSubregToReg())
     return true;
 
   bool isCheap = false;