cmd/asm/internal/arch: improve the comment of function IsMIPSMUL

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>

src/cmd/asm/internal/arch/mips.go

@@ -33,7 +33,7 @@ func IsMIPSCMP(op obj.As) bool {
 }
 
 // IsMIPSMUL reports whether the op (as defined by an mips.A* constant) is
-// one of the MUL/DIV/REM instructions that require special handling.
+// one of the MUL/DIV/REM/MADD/MSUB instructions that require special handling.
 func IsMIPSMUL(op obj.As) bool {
 	switch op {
 	case mips.AMUL, mips.AMULU, mips.AMULV, mips.AMULVU,