[GlobalISel] Add legalization support for non-power-2 loads and stores

[GlobalISel] Add legalization support for non-power-2 loads and stores

Legalize things like i24 load/store by splitting them into smaller power of 2 operations.

This matches how SelectionDAG handles these operations.

Differential Revision: https://reviews.llvm.org/D59971

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358613 91177308-0d34-0410-b5e6-96231b3b80d8