As #19357 reported,

TST has 3 different sub forms
TST $imme, Rx
TST Ry << imme, Rx
TST Ry << Rz, Rx

just like CMP/CMN/TEQ has. But current arm assembler assembles all TST
instructions wrongly. This patch fixes it and adds more tests.

Fixes #19357

Change-Id: Iafedccfaab2cbb2631e7acf259837a782e2e8e2f
Reviewed-on: https://go-review.googlesource.com/37662
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>