[ObjCARC] Do not move a release that has the clang.imprecise_release tag

[ObjCARC] Do not move a release that has the clang.imprecise_release tag
above PHI instructions.

ARC optimizer has an optimization that moves a call to an ObjC runtime
function above a phi instruction when the phi has a null operand and is
an argument passed to the function call. This optimization should not
kick in when the runtime function is an objc_release that releases an
object with precise lifetime semantics.

rdar://problem/34959669

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315914 91177308-0d34-0410-b5e6-96231b3b80d8