[Xtensa] fix up some error handling issues

[Bob Wilson <bwilson at tensilica dot com>]

I'm committing this patch on mainline to fix some problems with error  
handling in the assembler and linker.  The various  
"xtensa_opcode_is_*" functions return 0/1 with -1 on error, but many  
of the calls to these functions were only checking for non-zero  
results.  The most serious problem fixed here is a comparison in the  
"check_loop_aligned" function that was reversed, but it appears that  
the only effect of this bug was overly pessimistic alignment.  The  
rest of the changes in bfd are to speed up the error checking in the  
"compute_ebb_proposed_actions" function.  Previously the instruction  
was being decoded multiple times, but I've changed it to move the  
decoding in-line so that it is only done once.  I've tested these  
changes by using them at Tensilica for a few weeks and by running the  
testsuites with an xtensa-elf target.

bfd ChangeLog:

2006-05-15  David Heine  <dlheine@tensilica.com>
            Bob Wilson  <bob.wilson@acm.org>

        * elf32-xtensa.c (check_loop_aligned): Fix reversed check for
        undefined opcode.  Clean up assertions.
        (narrow_instruction, widen_instruction): Remove "do_it"  
parameters.
        Factor most of the code into separate functions....
        (can_narrow_instruction, can_widen_instruction): New.
        (prev_instr_is_a_loop): New.
        (compute_ebb_proposed_actions): Combine error handling code for
        decode errors.  Replace call to insn_decode_len with inline  
code.
        Use can_narrow_instruction and can_widen_instruction.   
Handle errors
        from call to xtensa_opcode_is_loop.
        (relax_section): Adjust calls to narrow_instruction and
        widen_instruction.

gas ChangeLog:

2006-05-15  Bob Wilson  <bob.wilson@acm.org>

        * config/tc-xtensa.c (is_direct_call_opcode,  
is_branch_jmp_to_next,
        xg_assemble_vliw_tokens, xtensa_mark_narrow_branches,
        xtensa_fix_short_loop_frags, is_local_forward_loop,  
relax_frag_immed):
        Handle errors from calls to xtensa_opcode_is_* functions.

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