[SimplifyCFG] Improve SwitchToLookupTable optimization

Try to use the original value as an index (in the lookup table)
in more cases (to avoid one subtraction and shorten the dependency chain)
(https://github.com/llvm/llvm-project/issues/56189).

Test plan:
1/ ninja check-all
2/ bootstrapped LLVM + Clang pass tests

Differential revision: https://reviews.llvm.org/D128897