Fix handling v-if along with v-for on a template tag

[simplesmiler]

Proposed fix for #2657

[yyx990803]

this could be simpler, since v-for and v-if are the only ones that can work on <template>, and v-for has higher priority than v-if, it seems v-if is the only thing that we need to check.

[paulpflug]

I'm unsure if this relates: while working on #2619, I noticed, that v-for is evaluated before slot, besides the priority says otherwise.
Could have the same origin? (although it is handy to have v-for before slot)

[simplesmiler]

@paulpflug are you talking about something like <slot v-for="...">? I don't think it's related, no.

[paulpflug]

@simplesmiler fine. It just attracted my attention that in both cases priorities don't work (the way I understand them 😄)

[simplesmiler]

@paulpflug this one has to do with the fact that <template> loses all attributes when being turned into a fragment factory 😉