[ARM] Add bitcast/extract_subvec. of fp16 vectors

Summary:
This patch adds some basic operations for fp16
vectors, such as bitcast from fp16 to i16,
required to perform extract_subvector (also added
here) and extract_element.

Reviewers: SjoerdMeijer, DavidSpickett, t.p.northover, ostannard

Reviewed By: ostannard

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60618

llvm-svn: 359433