Use faulting loads instead of test/jeq to do nil checks.
Fold nil checks into a following load/store if possible.

Makes binaries about 2% smaller.

Change-Id: I54af0f0a93c853f37e34e0ce7e3f01dd2ac87f64
Reviewed-on: https://go-review.googlesource.com/16287Reviewed-by: David Chase <drchase@google.com>