cmd/compile/internal/ir: explicit Pos for New{Bool,Int,String}

cmd/compile/internal/ir: explicit Pos for New{Bool,Int,String}

Stop depending on base.Pos for these.

Change-Id: I58dea44f8141eb37b59a6e9f7db0c6baa516ad93
Reviewed-on: https://go-review.googlesource.com/c/go/+/472296
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>