Don't force unused reg arg to the stack. (#44555)

* Don't force unused reg arg to the stack.

Author: CarolEidt

src/coreclr/src/jit/lsra.cpp

@@ -5596,8 +5596,13 @@ void LinearScan::allocateRegisters()
  {
  allocate = false;
  }
- else if (refType == RefTypeParamDef && varDsc->lvRefCntWtd() <= BB_UNITY_WEIGHT)
+ else if (refType == RefTypeParamDef && (varDsc->lvRefCntWtd() <= BB_UNITY_WEIGHT) &&
+ (!currentRefPosition->lastUse || (currentInterval->physReg == REG_STK)))
  {
+ // If this is a low ref-count parameter, and either it is used (def is not the last use) or it's
+ // passed on the stack, don't allocate a register.
+ // Note that if this is an unused register parameter we don't want to set allocate to false because that
+ // will cause us to allocate stack space to spill it.
  allocate = false;
  }
  else if ((currentInterval->physReg == REG_STK) && nextRefPosition->treeNode->OperIs(GT_BITCAST))