This is a minor improvement for the core_list_find routine in coremark.


Basically for riscv, and likely other targets, we can end up with an 
unconditional jump to a return statement.    This is a result of 
compensation code created by bb-reorder, and no jump optimization pass 
runs after bb-reorder to clean this stuff up.

This patch utilizes preexisting code to identify suitable branch targets 
as well as preexisting code to emit a suitable return, so it's pretty 
simple.  Note that when we arrange to do this optimization, the original 
return block may become unreachable. So we conditionally call 
delete_unreachable_blocks to fix that up.

This triggers ~160 times during an x86_64 bootstrap.  Naturally it 
bootstraps and regression tests on x86_64.

I've also bootstrapped this on riscv64, regression testing with qemu 
shows some regressions, but AFAICT they're actually qemu bugs with 
signal handling/delivery -- qemu user mode emulation is not consistently 
calling user defined signal handlers.  Given the same binary, sometimes 
they'll get called and the test passes, other times the handler isn't 
called and the test (of course) fails. I'll probably spend some time to 
try and chase this down for the sake of making testing easier.


OK for the trunk?


Jeff