exp/ssa: cross off a few remaining TODO issues.

- append: nothing to do (nonsemantic change).
- delete: now performs correct conversion (+ test).
- emitCompare: nothing to do.
- emitArith (shifts): nothing to do (+ test).
- "banish untyped types": give up on that.
- real, imag: now do correct conversions.
- added comment to interp.go re zero-size values.

R=gri
CC=golang-dev
https://codereview.appspot.com/7391046