bcache: Delete some slower inline asm

Never saw a profile of bset_search_tree() where it wasn't bottlenecked
on memory until I got my new Haswell machine, but when I tried it there
it was suddenly burning 20% of the cpu in the inner loop on shrd...

Turns out, the version of shrd that takes 64 bit operands has a 9 cycle
latency. hah.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>