[InstrRef][nfc] Remove usage of unique_ptrs of arrays (#74203)

These are usually difficult to reason about, and they were being used to
pass raw pointers around with array semantic (i.e., we were using
operator [] on raw pointers). To put it in InstrRef terminology: we were
passing a pointer to a ValueTable but using it as if it were a
FuncValueTable.

These could have easily been SmallVectors, which now allow us to have
reference semantics in some places, as well as simpler initialization.

In the future, we can use even more pass-by-reference with some extra
changes in the code.