[lld-macho][nfc] Simplify MarkLive.cpp using `if constexpr`

No significant perf diff, as expected.

             base           diff           difference (95% CI)
  sys_time   1.722 ± 0.030  1.727 ± 0.027  [  -0.6% ..   +1.2%]
  user_time  5.081 ± 0.032  5.087 ± 0.030  [  -0.2% ..   +0.4%]
  wall_time  6.008 ± 0.056  6.029 ± 0.053  [  -0.1% ..   +0.8%]
  samples    25             37

Reviewed By: #lld-macho, oontvoo, thakis, BertalanD

Differential Revision: https://reviews.llvm.org/D133014