[IR] Change std::sort to llvm::sort in response to r327219
r327219 added wrappers to std::sort which randomly shuffle the container before
sorting. This will help in uncovering non-determinism caused due to undefined
sorting order of objects having the same key.
To make use of that infrastructure we need to invoke llvm::sort instead of
Note: This patch is one of a series of patches to replace *all* std::sort to
llvm::sort. Refer D44363 for a list of all the required patches.