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.
We're starting to creep awfully close to this. Downstream we have 5 extra MVTs (though could get away with just 3), and Arm's Morello fork of CHERI LLVM adds another one (but we should really have more than one were we to have it in CHERI LLVM). Is there a plan for what to do when this limit finally gets hit? Because the slow but steady explosion of vector types is causing me some amount of worry for us downstream...