As discussion in issue #19141, the addend should be the third
argument of MULA. This patch fixes it in both the front end
and the back end of the assembler. And also tests are added to
the encoding test.

Fixes #19141

Change-Id: Idbc6f338b8fdfcad97a135f27a98c5b375b27d43
Reviewed-on: https://go-review.googlesource.com/42028
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>