Apply likely()/unlikely() to allocation/deallocation fast paths.
Fix irallocx_prof() sample logic.

Fix irallocx_prof() sample logic to only update the threshold counter
after it knows what size the allocation ended up being.  This regression
was caused by 6e73dc194ee9682d3eacaf725a989f04629718f7 (Fix a profile
sampling race.), which did not make it into any releases prior to this
fix.