This changes the ABI used on 32-bit x86 for passing vector arguments.
Historically, clang passes the first 4 vector arguments in-register, and additional vector arguments on the stack, regardless of platform. That is different from the behavior of gcc, icc, and msvc, all of which pass only the first 3 arguments in-register.
The 3-register convention is documented, unofficially, in Agner's calling convention guide, and, officially, in the recently released version 1.0 of the i386 psABI.
Darwin is kept as is because the OS X ABI Function Call Guide explicitly documents the current (4-register) behavior.
This fixes PR21510
Differential revision: http://reviews.llvm.org/D9644
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237682 91177308-0d34-0410-b5e6-96231b3b80d8
5440f2198d