[x86-64 ABI] Fix for PR23082: an assertion failure when passing/returning a wrapper...

[x86-64 ABI] Fix for PR23082: an assertion failure when passing/returning a wrapper union in a full YMM register.

This patch fixes an assertion failure in method
'X86_64ABIInfo::GetByteVectorType'.

Method 'GetByteVectorType' (in TargetInfo.cpp) is responsible
for mapping a QualType 'Ty' (for an argument or return value) to an LLVM IR
type that, according to the ABI, must be passed in a XMM/YMM vector register.

When selecting the IR vector type, method 'GetByteVectorType' always tries to
choose the "best" IR vector type for the 'Ty' in input. In particular, if Ty
is a wrapper structure, it keeps unwrapping it until it finds a vector type VTy.
That VTy is the "preferred IR type".

However, function 'isSingleElementStructure' (used to unwrap structures) does
not know how to look through union types. So, before this patch, if Ty was in
a nest of wrapper structures with at least two union types, we would have
triggered an assertion failure (added at revision 230971).

With this patch, if method 'GetByteVectorType' fails to find the preferred
vector type, we just return a valid (although potentially 'less friendly')
vector type based on the type size. So, rather than asserting on an 'unexpected'
'Ty' in input, we conservatively return vector type <2 x double> if Ty is 16
bytes, or <4 x double> if Ty is 32 bytes.

Differential Revision: http://reviews.llvm.org/D10190

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@238861 91177308-0d34-0410-b5e6-96231b3b80d8