cmd/compile: -S reports NOP as WORD $0 on mips and mips64

[minux]

go tool compile -S reports NOPs in delay slots as "WORD $0",
and what's more, there are extra zero-length NOP pseduo
instructions in the output, which is very confusing.

e.g.

$ GOARCH=mips go tool compile -S $GOROOT/test/helloworld.go
"".main t=1 size=104 args=0x0 locals=0x8
        0x0000 00000 (../test/helloworld.go:11) TEXT    "".main(SB), $8-0
        0x0000 00000 (../test/helloworld.go:11) MOVW    8(g), R1
        0x0004 00004 (../test/helloworld.go:11) SGTU    R29, R1, R1
        0x0008 00008 (../test/helloworld.go:11) BNE     R1, 36
        0x000c 00012 (../test/helloworld.go:11) WORD    $0
        0x0010 00016 (../test/helloworld.go:11) MOVW    R31, R3
        0x0014 00020 (../test/helloworld.go:11) CALL    runtime.morestack_noctxt(SB)
        0x0018 00024 (../test/helloworld.go:11) WORD    $0   // a real NOP in delay slot
        0x001c 00028 (../test/helloworld.go:11) JMP     0
        0x0020 00032 (../test/helloworld.go:11) WORD    $0  // a real NOP in delay slot
        0x0024 00036 (../test/helloworld.go:11) NOP   // a pseudo NOP
        0x0024 00036 (../test/helloworld.go:11) MOVW    R31, -12(R29)
        0x0028 00040 (../test/helloworld.go:11) ADDU    $-12, R29
        0x002c 00044 (../test/helloworld.go:11) FUNCDATA        $0, gclocals·2002e13acf59079a1a5782c918894579(SB)
        0x002c 00044 (../test/helloworld.go:11) FUNCDATA        $1, gclocals·2002e13acf59079a1a5782c918894579(SB)
        0x002c 00044 (../test/helloworld.go:12) PCDATA  $0, $0

I propose that we don't emit pseudo NOP (NOPs that are added by compiler but
discard by cmd/internal/obj) and replace delay slot filling NOPs as "NOP".

[minux]

/cc @cherrymui.

[cherrymui]

The zero-sized pseudo NOP are printed on all architecture. We probably don't want to change it for MIPS.

CL https://go-review.googlesource.com/c/40297/ changes hardware NOP from WORD $0 to NOOP.

[gopherbot]

CL https://golang.org/cl/40297 mentions this issue.