[ARM] Mark VSETLNi32 with the InsertSubreg property and implement the related
target hook.

This patch teaches the compiler that:
dX = VSETLNi32 dY, rZ, imm
is the same as:
dX = INSERT_SUBREG dY, rZ, translateImmToSubIdx(imm)

<rdar://problem/12702965>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216143 91177308-0d34-0410-b5e6-96231b3b80d8