resource.cc: Replace calls to find_basic_block with cfgrtl BLOCK_FOR_INSN

resource.cc: Replace calls to find_basic_block with cfgrtl BLOCK_FOR_INSN

...and call compute_bb_for_insn in init_resource_info and
free_bb_for_insn in free_resource_info.

I put a gcc_unreachable in that else-clause for a failing
find_basic_block in mark_target_live_regs after the comment that says:

    /* We didn't find the start of a basic block.  Assume everything
       in use.  This should happen only extremely rarely.  */
    SET_HARD_REG_SET (res->regs);

and found that it fails not extremely rarely but extremely early in
the build (compiling libgcc).

That kind of pessimization leads to suboptimal delay-slot-filling.
Instead, do like many machine_dependent_reorg passes and call
compute_bb_for_insn as part of resource.cc initialization.

After this patch, there's a whole "if (b != -1)" conditional that's
dominated by a gcc_assert (b != -1).  I separated that, as it's a NFC
whitespace patch that hampers patch readability.

Altogether this improved coremark performance for CRIS at -O2
-march=v10 by 0.36%.

	* resource.cc: Include cfgrtl.h.  Use BLOCK_FOR_INSN (insn)->index
	instead of calling find_basic_block (insn).  Assert for not -1.
	(find_basic_block): Remove function.
	(init_resource_info): Call compute_bb_for_insn.
	(free_resource_info): Call free_bb_for_insn.