The current code gets shift arguments value from prog.From3.Offset.
But prog.From3.Offset is not assigned the shift arguments value in
instructions assemble process.

The fix calls movcon() function to get the correct value.

Uncomment the movk/movkw  cases.

Fixes #21398
Change-Id: I78d40c33c24bd4e3688a04622e4af7ddb5333fa6
Reviewed-on: https://go-review.googlesource.com/54990
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>