[llvm] [DAG] Fold build_vector(build_pair()) patterns. (PR #88261)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> What would we get if we combined a single element build_vector+load into a load of the vector type instead?

I did consider this (as well as always replacing <1 x XX> buildvector with a bitcast in all cases not just build_pair) but wasn't sure if we should be creating illegal types so often.

https://github.com/llvm/llvm-project/pull/88261