Only lock unlocked registers in `rsUnspillRegPair`.
Only lock unlocked registers in `rsUnspillRegPair`.

`rsUnspillRegPair` needs to lock the hi/lo part of the reg pair when
unspilling the lo/hi part in order to ensure that the two parts of the
pair do not end up in the same register. However, the register to be
locked may already be locked if it is a multi-use register. In this case
`rsUnspillRegPair` does not need to re-lock the register.

Fixes #12910.

ruben-ayrapetyan/coreclr,cydhaselton/coreclr,yizhang82/coreclr,JonHanna/coreclr,wateret/coreclr,ruben-ayrapetyan/coreclr,ruben-ayrapetyan/coreclr,hseok-oh/coreclr,hseok-oh/coreclr,krk/coreclr,mmitche/coreclr,krk/coreclr,cshung/coreclr,cshung/coreclr,AlexGhiondea/coreclr,JosephTremoulet/coreclr,tijoytom/coreclr,krk/coreclr,poizan42/coreclr,russellhadley/coreclr,yizhang82/coreclr,russellhadley/coreclr,JosephTremoulet/coreclr,poizan42/coreclr,poizan42/coreclr,hseok-oh/coreclr,parjong/coreclr,JonHanna/coreclr,wtgodbe/coreclr,JonHanna/coreclr,yizhang82/coreclr,JosephTremoulet/coreclr,cydhaselton/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,cshung/coreclr,parjong/coreclr,ruben-ayrapetyan/coreclr,wateret/coreclr,hseok-oh/coreclr,JonHanna/coreclr,mskvortsov/coreclr,mmitche/coreclr,poizan42/coreclr,cydhaselton/coreclr,cydhaselton/coreclr,botaberg/coreclr,mskvortsov/coreclr,krk/coreclr,dpodder/coreclr,AlexGhiondea/coreclr,AlexGhiondea/coreclr,mskvortsov/coreclr,dpodder/coreclr,rartemev/coreclr,wtgodbe/coreclr,JosephTremoulet/coreclr,tijoytom/coreclr,mmitche/coreclr,rartemev/coreclr,rartemev/coreclr,rartemev/coreclr,rartemev/coreclr,JonHanna/coreclr,yizhang82/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,russellhadley/coreclr,kyulee1/coreclr,mskvortsov/coreclr,wtgodbe/coreclr,botaberg/coreclr,AlexGhiondea/coreclr,poizan42/coreclr,yizhang82/coreclr,wateret/coreclr,JonHanna/coreclr,cshung/coreclr,russellhadley/coreclr,cshung/coreclr,hseok-oh/coreclr,russellhadley/coreclr,poizan42/coreclr,yizhang82/coreclr,AlexGhiondea/coreclr,cshung/coreclr,botaberg/coreclr,kyulee1/coreclr,mmitche/coreclr,mskvortsov/coreclr,parjong/coreclr,mmitche/coreclr,kyulee1/coreclr,JosephTremoulet/coreclr,kyulee1/coreclr,wateret/coreclr,krk/coreclr,dpodder/coreclr,ruben-ayrapetyan/coreclr,parjong/coreclr,hseok-oh/coreclr,krk/coreclr,parjong/coreclr,wateret/coreclr,tijoytom/coreclr,dpodder/coreclr,botaberg/coreclr,rartemev/coreclr,russellhadley/coreclr,tijoytom/coreclr,cydhaselton/coreclr,cydhaselton/coreclr,AlexGhiondea/coreclr,botaberg/coreclr,tijoytom/coreclr,dpodder/coreclr,botaberg/coreclr,kyulee1/coreclr,mmitche/coreclr,mskvortsov/coreclr,parjong/coreclr,wateret/coreclr,JosephTremoulet/coreclr,tijoytom/coreclr,kyulee1/coreclr,dpodder/coreclr