port 231c8ac0 (r40522)

  original commit message:
  Loads already used source position elimination to avoid unnecessary hole checks,
  but for reasons unknown stores did not. This CL corrects that, making full-codegen's
  hole elimination equivalent to ignition's.

  Also introduced a HoleCheckMode enum class to avoid more bool flags and updated
  VariableProxy and BytecodeGenerator appropriately.

BUG=

Review-Url: https://codereview.chromium.org/2444823002
Cr-Commit-Position: refs/heads/master@{#40527}