Fix #7092
Fix #7092

The problem is genCodeForInitBlkUnroll() is trying to generate:
```
mov byte ptr [edi], si
```
which is not legal: ebp/esi/edi/esp don't have byte register versions on x86.

This change requires the source register to be an x86 byteable register in
this case.
Note that legacy JIT generates stosb for this test.

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