[llvm] [DAG] Support soft half promotion of insert_vector_elt (PR #194994)

[Philip Reames via llvm-commits]

preames wrote:

@topperc This one is for you.  I hit this originally while messing with the scalable unrolling stuff.  Initially it looked like a pretty straight forward patch, but the more I look at the legalization of the 16b float types without the corresponding extensions, the less sure I am I understand any of it.  I figure you probably have the best grasp, and can tell me whether this is actually useful or if I've got something backwards.

Couple notes for consideration:
1) If _neither_ are legal we fit an entirely different codepath.  This seems non-ideal, but starting to untangle that got complicated fast.  
2) Some of the legalization results for V look a bit suspicious (memory offsets changing between rv32/64?).  Am I testing something which is just ill defined?

https://github.com/llvm/llvm-project/pull/194994