[AVX] When joining two XMM registers into a YMM register, make sure that the

[AVX] When joining  two XMM registers into a YMM register, make sure that the
lower XMM register gets in first. This will allow the SUBREG pattern to
elliminate the first vector insertion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137310 91177308-0d34-0410-b5e6-96231b3b80d8