[llvm] [LoongArch] Override cost hooks to expose more DAG combine opportunities (PR #157824)

[via llvm-commits]

zhaoqi5 wrote:

> Are there any test cases that have benefited from this?

Mark extract floating point element from index 0 as cheap will lead to scalarize. Currently, we use float loads to get the scalar floating point constant, but use code sequence to generate the constants in vectors. So scalarization will get worse code in some cases.

A new pr (https://github.com/llvm/llvm-project/pull/158050) was just pushed to change the way of getting fp constant. After it is approved, I will find some tests for this commit.

https://github.com/llvm/llvm-project/pull/157824