cql3: fix overzealous move in modification_statement::get_mutations()
cql3: fix overzealous move in modification_statement::get_mutations()

'keys' and 'prefix' are used twice in the same expression, and as the language
does not guarantee any ordering in this case, any moves are illegal.

Get rid of them.

aruanruan/scylla,tempbottle/scylla,glommer/scylla,guiquanz/scylla,shaunstanislaus/scylla,phonkee/scylla,aruanruan/scylla,victorbriz/scylla,respu/scylla,senseb/scylla,bowlofstew/scylla,senseb/scylla,dwdm/scylla,victorbriz/scylla,tempbottle/scylla,duarten/scylla,kjniemi/scylla,aruanruan/scylla,glommer/scylla,justintung/scylla,bowlofstew/scylla,eklitzke/scylla,raphaelsc/scylla,rluta/scylla,linearregression/scylla,capturePointer/scylla,eklitzke/scylla,justintung/scylla,gwicke/scylla,phonkee/scylla,shaunstanislaus/scylla,capturePointer/scylla,kjniemi/scylla,capturePointer/scylla,acbellini/scylla,acbellini/scylla,guiquanz/scylla,avikivity/scylla,eklitzke/scylla,respu/scylla,wildinto/scylla,asias/scylla,respu/scylla,raphaelsc/scylla,stamhe/scylla,tempbottle/scylla,wildinto/scylla,kjniemi/scylla,duarten/scylla,stamhe/scylla,shaunstanislaus/scylla,senseb/scylla,avikivity/scylla,gwicke/scylla,acbellini/scylla,phonkee/scylla,linearregression/scylla,glommer/scylla,victorbriz/scylla,wildinto/scylla,duarten/scylla,scylladb/scylla,guiquanz/scylla,stamhe/scylla,scylladb/scylla,scylladb/scylla,rluta/scylla,avikivity/scylla,kangkot/scylla,rentongzhang/scylla,dwdm/scylla,asias/scylla,kangkot/scylla,justintung/scylla,dwdm/scylla,raphaelsc/scylla,linearregression/scylla,kangkot/scylla,scylladb/scylla,rentongzhang/scylla,rentongzhang/scylla,rluta/scylla,asias/scylla,gwicke/scylla,bowlofstew/scylla