[DAGCombine][ARM] Enable extending masked loads

Add generic DAG combine for extending masked loads.

Allow us to generate sext/zext masked loads which can access v4i8,
v8i8 and v4i16 memory to produce v4i32, v8i16 and v4i32 respectively.

Differential Revision: https://reviews.llvm.org/D68337

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375085 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed