LegalizeDAG: Support promoting [US]DIV and [US]REM operations

Summary:
AMDGPU will need this one i16 is added as a legal type.  This is tested by:

test/CodeGen/AMDGPU/sdiv.ll
test/CodeGen/AMDGPU/sdivrem24.ll
test/CodeGen/AMDGPU/udiv.ll
test/CodeGen/AMDGPU/udivrem24.ll

Reviewers: bogner, efriedma

Subscribers: efriedma, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D25699

llvm-svn: 285199