cmd/compile: inline convI2E

cmd/compile: inline convI2E

It's pretty simple.  For:
  e = (interface{})(i)
Do:
  tmp = i.itab
  if tmp != nil {
    tmp = tmp.typ_ // load type from itab
  }
  e = eface{tmp, i.data}

It is smaller and faster than calling the runtime.

Change-Id: I0ad27f62f4ec0b6cd53bc8530e4da0eae3e67a6c
Reviewed-on: https://go-review.googlesource.com/31260
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>