[vm/compiler] Do not count UnboxedConstant instructions in inliner

These instructions are often folded into indexing instructions.
Also, we do not want unboxing decisions to hinder inlining.

Fixes the regression of NNBD version of ParticleSystemUpdate
benchmark in AOT mode after https://dart-review.googlesource.com/c/sdk/+/150300

ParticleSystemUpdate +11-14%

Change-Id: I9707137195b8fe8550985fd972c02dcc59ce9239
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153540
Commit-Queue: Alexander Markov <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: alexmarkov

runtime/vm/compiler/backend/inliner.cc

@@ -194,6 +194,12 @@ class GraphInfoCollector : public ValueObject {
         if (current->IsRedefinition()) {
           continue;
         }
+        // UnboxedConstant is often folded into the indexing
+        // instructions (similar to Constant instructions which
+        // belong to initial definitions and not counted here).
+        if (current->IsUnboxedConstant()) {
+          continue;
+        }
         ++instruction_count_;
         // Count inputs of certain instructions as if separate PushArgument
         // instructions are used for inputs. This is done in order to