Fix a variety of bugs with nil-receiver checks when targeting

non-Darwin ObjC runtimes:

- Use the same logic the Darwin runtime does for inferring that a
  receiver is non-null and therefore doesn't require null checks.
  Previously we weren't skipping these for non-super dispatch.

- Emit a null check when there's a consumed parameter so that we can
  destroy the argument if the call doesn't happen.  This mostly
  involves extracting some common logic from the Darwin-runtime code.

- Generate a zero aggregate by zeroing the same memory that was used
  in the method call instead of zeroing separate memory and then
  merging them with a phi.  This uses less memory and avoids unnecessary
  copies.

- Emit zero initialization, and generate zero values in phis, using
  the proper zero-value routines instead of assuming that the zero
  value of the result type has a bitwise-zero representation.