This uses a new ZIR inst `array_init_sent` (and a ref equivalent) to
represent array init expressions that terminate in a a sentinel value.

The sentienl value is the last value in the `MultiOp` payload. This
makes it a bit more awkward to deal with (lots of "len - 1") but makes
it so that the payload matches the fact that sentinels appear at the end
of arrays. However, this is not a hill I want to die on so if we want to
change it to index 0, I'm happy to do so.

This makes the following work properly:

    try expect(@TypeOf([_:0]u8{}) == [0:0]u8);