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.
John McCall