[vm] Improve packing of Sample.

TEST=ci
Change-Id: Iad37e5f7331d731f4bf0ec9978ac1d06dd8693d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/459640
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

Author: rmacnak-google

runtime/vm/profiler.h

@@ -386,8 +386,8 @@ class Sample {
   uword pc_array_[kPCArraySizeInWords];
   uword vm_tag_;
   uword user_tag_;
-  uint32_t state_;
   Sample* next_;
+  uint32_t state_;
   uint32_t allocation_identity_hash_;
 
   using HeadSampleBit = BitField<decltype(state_), bool, 0, 1>;