[X86] Fix -Wstring-conversion in X86InstrInfo.cpp (NFC)

/Users/jiefu/llvm-project/llvm/lib/Target/X86/X86InstrInfo.cpp:9794:12: error: implicit conversion turns string literal into bool: 'const char[25]' to 'bool' [-Werror,-Wstring-conversion]
    assert("It should not reach here");
    ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode13.1/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/assert.h:99:25: note: expanded from macro 'assert'
    (__builtin_expect(!(e), 0) ? __assert_rtn(__func__, __ASSERT_FILE_NAME, __LINE__, #e) : (void)0)
                      ~ ^
1 error generated.