[llvm] [SDAG] Construct constants via instructions if materialization is costly (PR #86659)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

For integers, we're talking about increasing the register pressure in integer registers by 2; that doesn't seem like a serious issue.    For vectors, we can just use byte splats instead?

https://github.com/llvm/llvm-project/pull/86659