Added comment to clarify why we r211040 choose to bail out of fast isel instead
of generating a more complicated relocation, and fix mislabelled register in the
comments of the asan test case.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211052 91177308-0d34-0410-b5e6-96231b3b80d8