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>

src/cmd/internal/obj/x86/asm6.go

@@ -1864,10 +1864,14 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 		if p.As == AADJSP {
 			p.To.Type = obj.TYPE_REG
 			p.To.Reg = REG_SP
+			// Generate 'ADDQ $x, SP' or 'SUBQ $x, SP', with x positive.
+			// One exception: It is smaller to encode $-0x80 than $0x80.
+			// For that case, flip the sign and the op:
+			// Instead of 'ADDQ $0x80, SP', generate 'SUBQ $-0x80, SP'.
 			switch v := p.From.Offset; {
 			case v == 0:
 				p.As = obj.ANOP
-			case v < 0:
+			case v == 0x80 || (v < 0 && v != -0x80):
 				p.As = spadjop(ctxt, AADDL, AADDQ)
 				p.From.Offset *= -1
 			default: