[CodeGen][ARM] Fix FP16 vector coercion

Summary:
When a function argument or return type is a homogeneous aggregate
which contains an FP16 vector but the target does not support FP16
operations natively, the type must be converted into an array of
integer vectors by then front end (otherwise LLVM will handle FP16
vectors incorrectly by scalarizing them and promoting FP16 to float,
see https://reviews.llvm.org/D50507).

Currently the logic for checking whether or not a given homogeneous
aggregate contains FP16 vectors is incorrect: it only looks at the
type of the first vector.

This patch fixes the issue by adding a new method
ARMABIInfo::containsAnyFP16Vectors and using it. The traversal logic
of this method is largely the same as in
ABIInfo::isHomogeneousAggregate.

Reviewers: eli.friedman, olista01, ostannard

Reviewed By: ostannard

Subscribers: ostannard, john.brawn, javed.absar, kristof.beyls, pbarrio, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63437

llvm-svn: 363687