Split BytecodeGenerator::VisitSuspend into two pieces, one for
building the suspension code and one for resumption (these
are split into separate Build methods for convenience).
Each gets its own RegisterAllocationScope, which allows us to
reduce the register file size of the empty generator by 1.

For consistency, rename VisitGeneratorPrologue() to
BuildGeneratorPrologue() to match the names of the two
newly-created methods.

This relands the patch originally committed in
98927ea5, as the test failure
due to that change was a code flushing bug. Code flushing was
disabled in de4a4095.

R=rmcilroy@chromium.org

Bug: v8:6379
Change-Id: Ifb4deafea99693c0a4e8646cf4e9884c7374cfc6
Reviewed-on: https://chromium-review.googlesource.com/508814Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45406}