AM33 assembler: avoid using non-promoting AM30 insns

[Alexandre Oliva]

After the assembler patch I posted yesterday, that recognizes AM30
instructions in AM33 mode, a simple problem showed up.  Some AM30
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?