[AArch64] Add nvcast patterns for v2f32 -> v1f64

Summary: Constant stores of f32 values can create such NvCast nodes.

Reviewers: t.p.northover

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62285

llvm-svn: 361584