Port 231c8ac0

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.

R=adamk@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, bjaideep@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com
BUG=

Review-Url: https://codereview.chromium.org/2449473002
Cr-Commit-Position: refs/heads/master@{#40538}