cmd/internal/obj: drop Addr's Gotype field

cmd/internal/obj: drop Addr's Gotype field

The Gotype field is only used for ATYPE instructions. Instead of
specially storing the Go type symbol in From.Gotype, just store it in
To.Sym like any other 2-argument instruction would.

Modest reduction in allocations:

name       old alloc/op    new alloc/op    delta
Template      42.0MB ± 0%     41.8MB ± 0%  -0.40%         (p=0.000 n=9+10)
Unicode       34.3MB ± 0%     34.1MB ± 0%  -0.48%         (p=0.000 n=9+10)
GoTypes        122MB ± 0%      122MB ± 0%  -0.14%         (p=0.000 n=9+10)
Compiler       518MB ± 0%      518MB ± 0%  -0.04%         (p=0.000 n=9+10)

Passes toolstash -cmp.

Change-Id: I0e603266b5d7d4e405106a26369e22773a0d3a91
Reviewed-on: https://go-review.googlesource.com/31850
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>