The check of MADD&MSUB was added to the function IsMIPSMUL in
a previous commit, and the comments should also be updated.

Change-Id: I2d3da055d55b459b908714c542dff99ab5c6cf99
Reviewed-on: https://go-review.googlesource.com/c/go/+/171102Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>