cmd/internal/obj/x86: prep for next CL

Add unused (but initialized) from3 field to ytab, f3t to movtab.
Remove level of indentation in movtab processing.

Change-Id: I8475988f46b541ecaccf8d34608da8bef7d12e24
Reviewed-on: https://go-review.googlesource.com/6892Reviewed-by: Rob Pike <r@golang.org>