Move the ToBoolean elision in the BytecodeGenerator instead of the
peephole optimizer. Adds a TypeHint mechanism to the ExpressionResult
to enable passing of type hints through the ast visitor.

BUG=v8:6194

Change-Id: Ic55506ba11b213f7459250004d3f18cab04ee9b3
Reviewed-on: https://chromium-review.googlesource.com/467208
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44415}