[llvm-exegesis][NFC] Fix unused function 'IsMovRegToReg' warning

/scratch/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/unittests/tools/llvm-exegesis/X86/TargetTest.cpp:88:17: error: unused function 'IsMovRegToReg' [-Werror,-Wunused-function]
Matcher<MCInst> IsMovRegToReg(unsigned Opcode, int64_t Reg1, int64_t Reg2) {
1 error generated.