[X86, AVX2] Replace inserti128 and extracti128 intrinsics with generic shuffles

This should complete the job started in r231794 and continued in r232045:
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.

AVX2 introduced proper integer variants of the hacked integer insert/extract
C intrinsics that were created for this same functionality with AVX1.

This should complete the removal of insert/extract128 intrinsics.

The Clang precursor patch for this change was checked in at r232109.

llvm-svn: 232120