[CSSPGO] Silence -Wunused-but-set-variable warning without asserts (NFC)

/data/home/jiefu/llvm-project/llvm/lib/Transforms/IPO/SampleProfile.cpp:2189:8: error: variable 'IsFuncHashMismatch' set but not used [-Werror,-Wunused-but-set-variable]
  bool IsFuncHashMismatch = false;
       ^
1 error generated.