ssa_exprt::get_object_name: add fast path

[tautschnig]

In many cases (without field sensitivity actually: always) the object will just
be the original expression, no further processing is required. Thus add a fast
path that handles this case directly, which reduces the cost from 967 seconds
across all of SV-COMP's ReachSafety-ECA down to 143 seconds.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 148fd45).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/101009538