SharedFunctionInfo::name is always a String if present (never a Symbol),
so there's no need to first test it for IsName() before testing IsString().
This is enforced by a check in %FunctionSetName.

As a bonus, the code no longer looks invalid (unconditionally casting
a Name to a String). The logic came into this broken-looking state
accidentally in the flag cleanup CL https://codereview.chromium.org/2096933002/.

Review-Url: https://codereview.chromium.org/2162853003
Cr-Commit-Position: refs/heads/master@{#37914}