Revert "cmd/compile: de-virtualize interface calls"

This reverts commit 4e0c7c3f61.

Reason for revert: The presence-of-optimization test program is fragile, breaks under noopt, and might break if the Go libraries are tweaked.  It needs to be (re)written without reference to other packages.

Change-Id: I3aaf1ab006a1a255f961a978e9c984341740e3c7
Reviewed-on: https://go-review.googlesource.com/38097
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -483,11 +483,7 @@ func Main() {
 			}
 		}
 
-		// Just before compilation, compile itabs found on
-		// the right side of OCONVIFACE so that methods
-		// can be de-virtualized during compilation.
 		Curfn = nil
-		peekitabs()
 
 		// Phase 8: Compile top level functions.
 		// Don't use range--walk can add functions to xtop.