[llvm] [DAG] expandVecReduce - widen sub-legal vectors to not prematurely scalarize later reduction levels (PR #194672)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> @mstorsjo thanks - fix incoming

I've pushed the fix at #195251 - hit the same assert this morning in another x86 patch

https://github.com/llvm/llvm-project/pull/194672