[ARM] Add missing patterns for insert_subvector.

Summary: In some cases, shufflevector instruction can be transformed involving insert_subvector instructions. The ARM backend was missing some insert_subvector patterns, causing a failure during instruction selection. AArch64 has similar patterns.

Reviewers: t.p.northover, olista01, javed.absar, rengolin

Reviewed By: javed.absar

Subscribers: aemerson, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D36796

llvm-svn: 311543