sim: bfin: run-tests.sh: fix typo in usage exit
sim: bfin: handle invalid HLs encoding in dsp shift insns

For many of the 32bit dsp shift related insns, we were just ignoring the HLs
field.  The hardware does not though and will reject the insn if it's set
incorrectly.  Update the sim to match.
sim: bfin: note missing parallel handling of SEARCH

The SEARCH insn is an oddball when it comes to parallel usage.  It places a
big limit on what other insns it can run in parallel with, but we don't
currently track the amount of state needed to verify this (since no other insn
really requires this).  Add a note for now in case we get around to it.