[InstCombine] Refactor the simplification of pow() (NFC)

Refactor all cases dealing with `exp{,2,10}()` into one function in
preparation for D49273.  Otherwise, NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340061 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed