Revert r331816 and r331820 - [globalisel] Add a combiner helpers for extending...

Revert r331816 and r331820 - [globalisel] Add a combiner helpers for extending loads and use them in a pre-legalize combiner for AArch64

Reverting this to see if the clang-cmake-aarch64-global-isel and
clang-cmake-aarch64-quick bots are failing because of this commit.
We know it wasn't r331819.

llvm-svn: 331846