Recommit "[GlobalISel] Import patterns containing INSERT_SUBREG"

Recommit  "[GlobalISel] Import patterns containing INSERT_SUBREG"

I thought `llvm::sort` was stable for some reason but it's not.

Use `llvm::stable_sort` in `CodeGenTarget::getSuperRegForSubReg`.

Original patch: https://reviews.llvm.org/D66498

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370084 91177308-0d34-0410-b5e6-96231b3b80d8