Abseil benchmarks suggest that the conditional checks result in faster code (4-5x)
as they are compiled into conditional move instructions (cmov on x86).
Reviewed By: #libc, philnik, Mordante
Spies: pengfei, Mordante, philnik, libcxx-commits
Differential Revision: https://reviews.llvm.org/D125329
Laramie Leavitt