AM33 assembler: avoid using non-promoting AM30 insns

[Philip Blundell]

>instructions didn't allow for promotion of the immediate operands,
>whereas AM33 offers similar insns with wider immediate operands, so
>they must be allowed to promote.  Here's a patch that fixes this
>problem.  Ok to install?  Release branches?

This patch, and the previous one, are OK for the binutils release branch so 
long as they are approved for the trunk.

p.