[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>

llvm-svn: 216143