[llvm] [X86] Lower mathlib call ldexp into scalef when avx512 is enabled (PR #166839)

[Kavin Gnanapandithan via llvm-commits]

KavinTheG wrote:

Applied review suggestions:  
- getInsertVectorElt -> `SCALAR_TO_VECTOR`
- move scalar handling into switch. 

Changed: 
- ISD::FP_EXTEND/ISD::FP_ROUND -> `getFPExtendOrRound`
- ISD::SIGN_EXTEND -> `getSExtOrTrunc`


https://github.com/llvm/llvm-project/pull/166839