[vm/compiler/nnbd] Add slow path for loads of late fields without initializer

When loading a late field without initializer, we check if it was not
initialized and throw an exception. This change adds slow path (shared
if supported) for throwing LateInitializationError instead of calling
an initializer which would throw an exception. This improves register
allocation around accesses to such late fields.

In addition, this CL removes duplicated code for generating shared
stubs.

Size of Flutter gallery snapshot on armv8 -0.45%
(gzip -0.34%, brotli -0.41%)

Null-safe version of ParticleSystemUpdate benchmark on x64
JIT +9-17%, AOT +3-4%.

Change-Id: Ia12c1efec0871a2fd7acc6db75f255853af52d0f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/168993
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>