Fix IdentityPartitionMapping perf optimization (#18456)

## Summary & Motivation
#17703 removed partitions_def
from DefaultPartitionMapping, which these optimizations were relying on
existing.

We should probably remove that field from the base class now that it is
no longer reliably there and is only available on time based partition
mapping - but that is out of scope for this fix

## How I Tested These Changes
Dry run for an asset graph involving multi-partitions finishes quickly
again

python_modules/dagster/dagster/_core/definitions/partition_mapping.py

@@ -122,7 +122,7 @@ def get_upstream_mapped_partitions_result_for_partitions(
         if downstream_partitions_subset is None:
             check.failed("downstream asset is not partitioned")
 
-        if downstream_partitions_subset.partitions_def == upstream_partitions_def:
+        if downstream_partitions_def == upstream_partitions_def:
             return UpstreamPartitionsResult(downstream_partitions_subset, [])
 
         upstream_partition_keys = set(
@@ -150,7 +150,7 @@ def get_downstream_partitions_for_partitions(
         if upstream_partitions_subset is None:
             check.failed("upstream asset is not partitioned")
 
-        if upstream_partitions_subset.partitions_def == downstream_partitions_def:
+        if upstream_partitions_def == downstream_partitions_def:
             return upstream_partitions_subset
 
         upstream_partition_keys = set(upstream_partitions_subset.get_partition_keys())