GVN: Preserve invariant.load metadata

If both instructions to be replaced are marked invariant the resulting
instruction is invariant.

rdar://13358910

Fix by Erik Eckstein!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211801 91177308-0d34-0410-b5e6-96231b3b80d8