[X86, AVX] Replace vinsertf128 intrinsics with generic shuffles.

We want to replace as much custom x86 shuffling via intrinsics
as possible because pushing the code down the generic shuffle
optimization path allows for better codegen and less complexity
in LLVM.

This is the sibling patch for the LLVM half of this change:
http://reviews.llvm.org/D8086

Differential Revision: http://reviews.llvm.org/D8088

llvm-svn: 231792