cmd/compile: "illegal combination SRA ADDCON REG REG" on mips

[ALTree]

$ go version
go version go1.15.5 linux/amd64

$ gotip version
go version devel +f423d616b1 Fri Nov 13 15:15:15 2020 +0000 linux/amd64

The following program:

package p

func f() {
	var i, j int
	_ = func() {
		i = 32
		j = j>>i | len([]int{})
	}
}

Crashes the go1.15.5 and tip compilers when built for mips:

$ GOARCH=mips go tool compile crash.go 

00020 (crash.go:7)	SRA	$-1, R2, R2
00020 (crash.go:7)	SRA	$-1, R2, R2
00020 (crash.go:7)	SRA	$-1, R2, R2
00020 (crash.go:7)	SRA	$-1, R2, R2
crash.go:7:5: illegal combination SRA ADDCON REG REG

The go1.14.12 compiler compiles it with no issues.

cc @cherrymui @randall77

[gopherbot]

Change https://golang.org/cl/270117 mentions this issue: cmd/compile: check unsigned value for SRA mips rule

[gopherbot]

Change https://golang.org/cl/270558 mentions this issue: cmd/compile: stop MOVW-ing -1 as SRA shift amount in mips