cmd/compile/internal/ir: panic in `IsSynthetic` on `openbsd/arm64`

#!watchflakes
post <- goos == "openbsd" && goarch == "arm64" && log ~ `panic: runtime error: invalid memory address or nil pointer dereference` && log ~ `^cmd/compile/internal/ir\.IsSynthetic`

Issue created automatically to collect these failures.

Example (log):

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x32509c]

goroutine 10 [running]:
cmd/compile/internal/ir.IsSynthetic(...)
	cmd/compile/internal/ir/node.go:68
cmd/compile/internal/ssa.BuildFuncDebug(0x83e600?, 0x4000a941a0, 0x40002172c0?, 0x9?, 0x4000b15400)
	cmd/compile/internal/ssa/debug.go:600 +0xb2c
cmd/compile/internal/ssagen.genssa(0x4000a941a0, 0x4000e38af0)
	cmd/compile/internal/ssagen/ssa.go:7327 +0xe00
cmd/compile/internal/ssagen.Compile(0x40004869a0, 0x40006e6000?)
	cmd/compile/internal/ssagen/pgen.go:197 +0x280
cmd/compile/internal/gc.compileFunctions.func5.1(0x40006e6060?)
	cmd/compile/internal/gc/compile.go:184 +0x3c
cmd/compile/internal/gc.compileFunctions.func3.1()
	cmd/compile/internal/gc/compile.go:166 +0x3c
created by cmd/compile/internal/gc.compileFunctions.func3 in goroutine 9
	cmd/compile/internal/gc/compile.go:165 +0x1d8
go tool dist: FAILED: /home/gopher/build/go/pkg/tool/openbsd_arm64/go_bootstrap install std: exit status 1

— watchflakes