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

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

> Really not sure about this - why not just use custom lowering in the RISCV target?

I was thinking that maybe other targets can benefit from this.

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