When a constant is both MOVCON (can fit into a MOV instruction)
and BITCON (can fit into a logical instruction), the assembler
chooses to use the MOVCON encoding, which is actually longer for
logical instructions. We add MBCON rules explicitly to make sure
it uses the BITCON encoding.

Updates #19857.

Change-Id: Ib9881be363cbc491ac2a0792b36b87e74eff34a8
Reviewed-on: https://go-review.googlesource.com/39652
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>