Switch statements introduce their own scope for cases, but this scope
is not necessarily executed in order, as the following function shows:

  switch (x) {
    case 1:
      let y = 1;
    case 2:
      y = 2;
    case 3:
      print(y);
  }

If x = 2 or x = 3, the code should throw a ReferenceError. However,
FullCodeGen's hole check elimination used the simple algorithm of
assuming that if the initializer was in the same scope, then it was
reached before the use, and therefore the hole check could be
eliminated.

This patch adds an extra bit to scopes, to track if they may
nonlinearly. The parser marks the scope that switch introduces as
nonlinear. FullCodeGen does not eliminate the hole check from
a scope which is nonlinear. This patch refactors FullCodeGen to
put the hole check elimination in one place, rather than in each
backend.

BUG=v8:3926
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1312613003

Cr-Commit-Position: refs/heads/master@{#30453}