[ARM] Mark VMOVRRD with the ExtractSubreg property and implement the related
target hook.

This patch teaches the compiler that:
rX, rY = VMOVRRD dZ
is the same as:
rX = EXTRACT_SUBREG dZ, ssub_0
rY = EXTRACT_SUBREG dZ, ssub_1

<rdar://problem/12702965>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216132 91177308-0d34-0410-b5e6-96231b3b80d8