[dev.regabi] cmd/compile: make sure mkcall* passed non-nil init

So next CL can pass temporaries assignments for function arguments in to
init instead of CallExpr.Rargs.

Passes toolstash -cmp.

Change-Id: I2c3cb6a63e8bf9d0418052b39c1db58050f71305
Reviewed-on: https://go-review.googlesource.com/c/go/+/284893


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>