Avoid repeated checks and context lookup in llvm::Instruction::getAAMetadata

Repeated call to Instruction::hasMetadata() and lookup in
getContext().pImpl->ValueMetadata are not needed when we always work on
the same instruction, avoid them.

This simple change leads to interesting and consistent speedups in
compile time, around -0.5% according to http://llvm-compile-time-tracker.com, see

        http://llvm-compile-time-tracker.com/compare.php?from=9708d88017d0c9adaea65a4f5a5b589b67f292e2&to=88482b9addd5917e9c5226847efeb5451c5f78c0&stat=instructions

Differential Revision: https://reviews.llvm.org/D136503