fix(compiler-core): should not remove slot node with v-else

[KermanX]

Previously, slot nodes with v-else or v-else-if directives were wrongly removed by the compiler. This caused issues like vuejs/language-tools#3466 and vuejs/language-tools#4443.

Previously, the following source code:

<Comp>
  <template v-if="a" #foo />
  <template v-else #foo />
</Comp>

Parse mode ✅	Compile mode ❌

Now, the compile mode's behavior is the same as the parse mode.