[ARM] Add a batch of MVE floating-point instructions.

[ARM] Add a batch of MVE floating-point instructions.

Summary:
This includes floating-point basic arithmetic (add/sub/multiply),
complex add/multiply, unary negation and absolute value, rounding to
integer value, and conversion to/from integer formats.

Reviewers: dmgreen, samparker, SjoerdMeijer, t.p.northover

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62675

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364013 91177308-0d34-0410-b5e6-96231b3b80d8