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

[Kavin Gnanapandithan via llvm-commits]

KavinTheG wrote:

Applied review suggestions
  - Kept Exp the same type (integer) after the switch statement.
    -  Decided it was easier to keep Exp to an int during the switch, and convert to fp after widening.  
  - Scalef/s only needs two arguments 
  - `getFPExtendOrRound` used directly. 

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