go/ssa/interp: fix tests on Go 1.7 and Go 1.8

This mostly reverts commit 1c59bc354db124c4a797df6930a19c7a3cf8008c
(CL 43350) which had fixed Go tip but broken Go 1.7 and Go 1.8
builders.

Change-Id: I4e7bdfafde74c9e730bb870b9db05b663b4f56a3
Reviewed-on: https://go-review.googlesource.com/43391
Reviewed-by: Alan Donovan <adonovan@google.com>
2 files changed