[globalisel] Add a combiner helpers for extending loads and use them in a pre-legaliz...

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

Summary: Depends on D45541

Reviewers: ab, aditya_nandakumar, bogner, rtereshin, volkan, rovka, javed.absar, aemerson

Subscribers: aemerson, rengolin, mgorny, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D45543

llvm-svn: 343521