[CostModel][ARM] Increase cost of insert/extract operations

Summary:
This change limits the minimum cost of an insert/extract
element operation to 2 in cases where this would result
in mixing of NEON and VFP code.

Reviewers: rengolin

Subscribers: mssimpso, aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D12030

llvm-svn: 245225