[AArch64] Add negative tests for the SIMD & FP LDP instructions.

LDP is unpredictable if the registers in the pair are identical, these tests check that we don't assemble instructions like that and error out instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213074 91177308-0d34-0410-b5e6-96231b3b80d8