Mark RetBuf as SIMD variable
Mark RetBuf as SIMD variable

I see changes where storeIndir is used now instead of
a BLK copy, which is an IR shape improvement.

This improves codegen for SIMD8, degrades it for SIMD12 (due
to encoding size differences with SSE instructions, though
the IR looks better), and is neutral for SIMD16, in the
VectorReturn test.

cshung/coreclr,krk/coreclr,parjong/coreclr,krytarowski/coreclr,qiudesong/coreclr,cshung/coreclr,yizhang82/coreclr,krytarowski/coreclr,yizhang82/coreclr,JosephTremoulet/coreclr,gkhanna79/coreclr,krk/coreclr,ragmani/coreclr,krytarowski/coreclr,mmitche/coreclr,tijoytom/coreclr,tijoytom/coreclr,mskvortsov/coreclr,ruben-ayrapetyan/coreclr,botaberg/coreclr,krytarowski/coreclr,wateret/coreclr,YongseopKim/coreclr,AlexGhiondea/coreclr,alexperovich/coreclr,yizhang82/coreclr,parjong/coreclr,alexperovich/coreclr,dpodder/coreclr,botaberg/coreclr,JosephTremoulet/coreclr,parjong/coreclr,botaberg/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,yizhang82/coreclr,dpodder/coreclr,poizan42/coreclr,wtgodbe/coreclr,AlexGhiondea/coreclr,jamesqo/coreclr,mmitche/coreclr,James-Ko/coreclr,sjsinju/coreclr,JonHanna/coreclr,cshung/coreclr,mmitche/coreclr,ruben-ayrapetyan/coreclr,JosephTremoulet/coreclr,jamesqo/coreclr,JonHanna/coreclr,qiudesong/coreclr,neurospeech/coreclr,hseok-oh/coreclr,tijoytom/coreclr,ruben-ayrapetyan/coreclr,mskvortsov/coreclr,cmckinsey/coreclr,ruben-ayrapetyan/coreclr,poizan42/coreclr,botaberg/coreclr,dpodder/coreclr,ragmani/coreclr,yeaicc/coreclr,cshung/coreclr,James-Ko/coreclr,botaberg/coreclr,krk/coreclr,dpodder/coreclr,yeaicc/coreclr,botaberg/coreclr,russellhadley/coreclr,yeaicc/coreclr,neurospeech/coreclr,tijoytom/coreclr,cshung/coreclr,sagood/coreclr,sjsinju/coreclr,russellhadley/coreclr,mmitche/coreclr,pgavlin/coreclr,neurospeech/coreclr,pgavlin/coreclr,krytarowski/coreclr,kyulee1/coreclr,yeaicc/coreclr,yeaicc/coreclr,YongseopKim/coreclr,rartemev/coreclr,kyulee1/coreclr,James-Ko/coreclr,sjsinju/coreclr,krk/coreclr,pgavlin/coreclr,wateret/coreclr,kyulee1/coreclr,wtgodbe/coreclr,cshung/coreclr,James-Ko/coreclr,alexperovich/coreclr,sagood/coreclr,jamesqo/coreclr,alexperovich/coreclr,kyulee1/coreclr,alexperovich/coreclr,wateret/coreclr,wtgodbe/coreclr,cydhaselton/coreclr,poizan42/coreclr,yizhang82/coreclr,pgavlin/coreclr,kyulee1/coreclr,neurospeech/coreclr,rartemev/coreclr,JonHanna/coreclr,poizan42/coreclr,sjsinju/coreclr,hseok-oh/coreclr,JosephTremoulet/coreclr,parjong/coreclr,wtgodbe/coreclr,jamesqo/coreclr,cmckinsey/coreclr,pgavlin/coreclr,YongseopKim/coreclr,YongseopKim/coreclr,cmckinsey/coreclr,YongseopKim/coreclr,tijoytom/coreclr,cmckinsey/coreclr,poizan42/coreclr,ragmani/coreclr,poizan42/coreclr,hseok-oh/coreclr,JosephTremoulet/coreclr,JonHanna/coreclr,alexperovich/coreclr,hseok-oh/coreclr,mskvortsov/coreclr,JonHanna/coreclr,cydhaselton/coreclr,mskvortsov/coreclr,krytarowski/coreclr,JosephTremoulet/coreclr,kyulee1/coreclr,wateret/coreclr,yeaicc/coreclr,qiudesong/coreclr,russellhadley/coreclr,hseok-oh/coreclr,hseok-oh/coreclr,ruben-ayrapetyan/coreclr,sjsinju/coreclr,qiudesong/coreclr,gkhanna79/coreclr,mskvortsov/coreclr,AlexGhiondea/coreclr,cydhaselton/coreclr,James-Ko/coreclr,ragmani/coreclr,rartemev/coreclr,neurospeech/coreclr,gkhanna79/coreclr,AlexGhiondea/coreclr,cmckinsey/coreclr,James-Ko/coreclr,JonHanna/coreclr,pgavlin/coreclr,gkhanna79/coreclr,mskvortsov/coreclr,tijoytom/coreclr,sagood/coreclr,yeaicc/coreclr,jamesqo/coreclr,ragmani/coreclr,russellhadley/coreclr,cmckinsey/coreclr,sagood/coreclr,AlexGhiondea/coreclr,ragmani/coreclr,russellhadley/coreclr,dpodder/coreclr,yizhang82/coreclr,krk/coreclr,rartemev/coreclr,AlexGhiondea/coreclr,russellhadley/coreclr,YongseopKim/coreclr,dpodder/coreclr,parjong/coreclr,cydhaselton/coreclr,wateret/coreclr,rartemev/coreclr,qiudesong/coreclr,cydhaselton/coreclr,krk/coreclr,sjsinju/coreclr,mmitche/coreclr,sagood/coreclr,jamesqo/coreclr,gkhanna79/coreclr,sagood/coreclr,gkhanna79/coreclr,wateret/coreclr,neurospeech/coreclr,parjong/coreclr,cmckinsey/coreclr,wtgodbe/coreclr,cydhaselton/coreclr,mmitche/coreclr,rartemev/coreclr,qiudesong/coreclr