Fix #7642.

Explicitly call convert on append/prepend/splice items to match 1st array elements. This causes a MethodError to be thrown before any array growing/shrinking happens. Fixes #7642.

cc: @kmsquire, in particular I think the `splice!` changes should do the trick, but I'd appreciate a look over to make sure I didn't miss something.
You moved the call to `length` to before `jl_array_grow_end`, at which point the length is different.