ARM64: extract a 32-bit subreg when selecting an inreg extend

After the load/store refactoring, we were sometimes trying to feed a
GPR64 into a 32-bit register offset operand. This failed in
copyPhysReg.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209566 91177308-0d34-0410-b5e6-96231b3b80d8