[clang] [llvm] [InstCombine] Combine llvm.sin/llvm.cos libcall pairs into llvm.sincos (PR #184760)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

Changes:
- Rebase
- Propagate `fpmath` to llvm.sincos correctly (was propagated to extract value due to LLVM limitation), thanks @arsenm!
- New testcase `sincos-fpmath.ll`, it separated from from `sincos.ll`, main reason is I want to check exact value of `fpmath` instead of `!fpmath [[META0:![0-9]+]]` which generated by auto update script.

https://github.com/llvm/llvm-project/pull/184760