runtime: make explicit nil check in heapSetTypeSmallHeader
This is another case very similar to CL 684015 and #74375.
In spans with type headers, mallocgc always writes to the page before
returning the allocated memory. This initial write is done by
runtime.heapSetTypeSmallHeader. Prior to the write, the compiler inserts
a nil check, implemented as a dummy instruction reading from memory.
On a freshly mapped page, this read triggers a page fault, mapping the
zero page read-only. Immediately afterwards, the write triggers another
page fault, copying to a writeable page and performing a TLB flush.
This problem is exacerbated as the process scales up. At GOMAXPROCS=6,
the tile38 sweet benchmark spends around 0.1% of cycles directly
handling these page faults. On the same machine at GOMAXPROCS=192, it
spends about 2.7% of cycles directly handling these page faults.
Replacing the read with an explicit nil check reduces the direct cost of
these page faults down to around 0.1% at GOMAXPROCS=192. There are
additional positive side-effects due to reduced contention, so the
overall time spent in page faults drops from around 12.8% to 6.8%. Most
of the remaining time in page faults is spent on automatic NUMA page
migration (completely unrelated to this issue).