Introduces a IsCompiledScope object which can be used to check whether a
function is compiled, and ensure it remains compiled for the lifetime
of the scope without being uncompiled by bytecode flushing. The Compile
functions are modified to take a scope so that calling code can ensure
the function remains compiled for the lifetime they require.

Also, don't allocate a feedback vector for asm-wasm code as this
is never used, and will be reallocated if the asm-wasm code fails to
instantiate the module and we fallback to regular JavaScript.

Also restructure Compiler::PostInstantiation() to allocate the feedback
vector once, and do the optimized code check before optimizing for
always opt.

BUG=v8:8395

Change-Id: I3f1a71143fcae3d1a0c01eefe91ebb4b8594221a
Reviewed-on: https://chromium-review.googlesource.com/c/1352295Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57971}