cmd/compile: test to make sure that OPmodify ops clobber flags

[randall77]

In CL 122556, we fixed a bug by marking ADDLmodify and friends as clobbering flags.
This issue is to add a test for that case.

[gopherbot]

Change https://golang.org/cl/124535 mentions this issue: cmd/compile: add test for OPmodify ops clobbering flags