[llvm] DAG: Consider __sincos_stret when deciding to form fsincos (PR #165169)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LGTM (though I'm not sure the same works for AArch64).

It works, there's just one regression from processing the undef 4th vector element 

https://github.com/llvm/llvm-project/pull/165169