[CodeGen] Move ARMCodegenPrepare to TypePromotion

Convert ARMCodeGenPrepare into a generic type promotion pass by:
- Removing the insertion of arm specific intrinsics to handle narrow
  types as we weren't using this.
- Removing ARMSubtarget references.
- Now query a generic TLI object to know which types should be
  promoted and what they should be promoted to.
- Move all codegen tests into Transforms folder and testing using opt
  and not llc, which is how they should have been written in the
  first place...

The pass searches up from icmp operands in an attempt to safely
promote types so we can avoid generating unnecessary unsigned extends
during DAG ISel.

Differential Revision: https://reviews.llvm.org/D69556

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