With Bytecode flushing, the a SharedFunctionInfo's bytecode might be flushed
while the compiler is expecting it to still exist. Rather than continually
getting the bytecode from the SFI, instead bottleneck the points where we get
BytecodeArray from SFIs and maintain an explicit strong reference to the
BytecodeArray from that point onwards to prevent flushing.

BUG=v8:8395

Change-Id: I6a18adec99402838690971eb37ee0617cdc15920
Reviewed-on: https://chromium-review.googlesource.com/c/1309763
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57536}