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.

Review-Url: https://codereview.chromium.org/2441543005
Cr-Commit-Position: refs/heads/master@{#40522}