[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 Clang half of this change:
http://reviews.llvm.org/D8088

Differential Revision: http://reviews.llvm.org/D8086

llvm-svn: 231794