[llvm] [DAG] Enable bitcast STLF for Constant/Undef (PR #172523)

[via llvm-commits]

Michael-Chen-NJU wrote:

Hi @RKSimon, @arsenm,

I have updated the patch to address all comments:

1. Used DAG.getExtractSubvector to simplify the node creation.

2. Cleaned up the tests by adding nounwind and removing unnecessary fast-math flags as suggested.

Regarding the STLF classification: Since there are no further objections to the "Branch B1 (Vector-to-Vector)" logic, I'll assume this scope is acceptable for this PR. I'm keeping the Branch B2 (Scalar-from-Vector) out for now specifically to avoid the DAG combiner loops I encountered during local testing.

**Please let me know if you'd prefer me to include the Branch B2 logic in this same PR, or if moving forward with this incremental fix first is preferred to ensure stability.**

If this looks good, could we move forward with merging this part first?

https://github.com/llvm/llvm-project/pull/172523