Tweak BumpPtrAllocator to benefit the hot path (#90571)

This takes the form of three consecutive but related changes:
- Mark the fast path of BumpPtrAllocator as likely-taken.
- Move the slow path of BumpPtrAllocator to a separate function.
- Mark the slow path of BumpPtrAllocator as noinline.

Overall, this saves geomean 0.4% userspace instructions on CTMark -O3,
and 0.98% on CTMark -O0 -g.


http://llvm-compile-time-tracker.com/compare.php?from=e1622e189e8c0ef457bfac528f90a7a930d9aad2&to=9eb53a4ed3af4a55e769ae1dd22d034b63d046e3&stat=instructions%3Au