Replace BYTE.. encodings with asm. This is possible due to asm
implementing more instructions and removal of
MOV $0, reg -> XOR reg, reg transformation from asm.

Change-Id: I011749ab6b3f64403ab6e746f3760c5841548b57
Reviewed-on: https://go-review.googlesource.com/97936
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>