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.

Bug: v8:6379
Change-Id: I08a617a44f99706cfff09bf86fb0a25a9cd6e032
Reviewed-on: https://chromium-review.googlesource.com/503593
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45318}