Allow AArch64FastISel to degrade graceully in the presence of an MVT::i128

Currently AArch64FastISel crashes if it tries to extend an integer into an
MVT::i128. This can happen by creating 128 bit integers like so:

  typedef unsigned int uint128_t __attribute__((mode(TI)));
  typedef int sint128_t __attribute__((mode(TI)));

This patch makes EmitIntExt check for their presence and then falls back to
SelectionDAG.

Tests included.

rdar://17516686

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212492 91177308-0d34-0410-b5e6-96231b3b80d8