[IRGen] Fix FixedArray of fixedSize 1 elt addring.
When a `FixedArray`'s fixed size is 1, it looks like `[1 x %Ty]`. Given
an array's address, performing an operation on each element's address
entail's indexing into the array to each element's index to produce an
element's address for each index. That is true even when the array
consists of a single element. In that case, produce an address for that
single element by indexing to index 0 into each passed-in array.
rdar://151726387
