Instead of being a hint, resultInArg0 is now enforced by regalloc.
This allows us to delete all the code from amd64/ssa.go which
deals with converting from a semantically three-address instruction
into some copies plus a two-address instruction.

Change-Id: Id4f39a80be4b678718bfd42a229f9094ab6ecd7c
Reviewed-on: https://go-review.googlesource.com/21816Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>