[llvm-profgen] Fix warnings when building without asserts [NFC]
Building without asserts we got:
../tools/llvm-profgen/ProfiledBinary.cpp:627:14: error: unused variable 'Err' [-Werror,-Wunused-variable]
627 | bool Err = MIA->evaluateBranch(Inst, Address, Size, Target);
| ^~~
../tools/llvm-profgen/ProfiledBinary.cpp:1172:14: error: unused variable 'TopProbe' [-Werror,-Wunused-variable]
1172 | auto TopProbe = TopLevelProbes.begin();
| ^~~~~~~~
2 errors generated.
Add [[maybe_unused]] to the variables just used in asserts.