cmd/internal/obj: remove F3t field from Prog

F3t was effectively a local variable.
Remove it.

This shrinks obj.Prog from 456 to 448 bytes,
which places it in a smaller malloc class.

This reduces the memory usage of the compiler
while compiling the rotate tests by ~2.75%.

Change-Id: I31cc9dd67269851a430b56bcc7d255c9349eb522
Reviewed-on: https://go-review.googlesource.com/10255Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/link.go

@@ -222,7 +222,6 @@ type Prog struct {
 	Scond    uint8
 	Back     uint8
 	Ft       uint8
-	F3t      uint8
 	Tt       uint8
 	Isize    uint8
 	Printed  uint8

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

@@ -2974,15 +2974,12 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 	if p.Ft == 0 {
 		p.Ft = uint8(oclass(ctxt, p, &p.From))
 	}
-	if p.F3t == 0 {
-		p.F3t = uint8(oclass(ctxt, p, &p.From3))
-	}
 	if p.Tt == 0 {
 		p.Tt = uint8(oclass(ctxt, p, &p.To))
 	}
 
 	ft := int(p.Ft) * Ymax
-	f3t := int(p.F3t) * Ymax
+	f3t := oclass(ctxt, p, &p.From3) * Ymax
 	tt := int(p.Tt) * Ymax
 
 	xo := obj.Bool2int(o.op[0] == 0x0f)