cmd/internal/obj/x86: slightly optimize ADJSP encoding
This shaves a few bytes off here and there.
file before after Δ %
buildid
2865992 2861896 -4096 -0.143%
pprof
14744060 14739964 -4096 -0.028%
trace
11680644 11676548 -4096 -0.035%
vet
8448240 8444144 -4096 -0.048%
Change-Id: I799034afabc06a37b535301cd1380d63b4461095
Reviewed-on: https://go-review.googlesource.com/c/go/+/168343
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <martisch@uos.de>