[ValueTypes] Define MVTs for v128i2/v64i4 as well as i2 and i4.

Adds MVT::v128i2, MVT::v64i4, and implied MVT::i2, MVT::i4.

Keeps MVT::i2, MVT::i4 lowering actions as `expand`, which should be
removed once targets set this explicitly.

Adjusts 11 lit tests to reflect slightly different behavior during
DAG combine.

Differential Revision: https://reviews.llvm.org/D125247