pa.md (conditional branches): Use the full displacement range for the branch target.

        * pa.md (conditional branches): Use the full displacement range
        for the branch target.  Update length computations to match current
        reality.
        (branch on bit patterns): Likewise.
        (decrement_and_branch_until_zero): Re-enable pattern.  Rewrite to
        be simpler and more efficient.  Also handle case where loop counter
        is in a FP register.

From-SVN: r4874