[AArch64] Improved bitfield instruction selection.

Summary:
For bitfield insert OR matching, check both operands for larger pattern
first before checking for smaller pattern.

Add pattern for unsigned bitfield insert-in-zero done with SHL+AND.

Resolves PR21631.

Reviewers: jmolloy, t.p.northover

Subscribers: aemerson, rengolin, llvm-commits, mcrosier

Differential Revision: http://reviews.llvm.org/D12908

llvm-svn: 248006