Simp set for if-then-else statements
Splits all if-then-else-expressions into multiple goals.
Given a goal of the form g (if p then x else y), split_ifs will produce
two goals: p ⊢ g x and ¬p ⊢ g y.
If there are multiple ite-expressions, then split_ifs will split them all,
starting with a top-most one whose condition does not contain another
ite-expression.
split_ifs at * splits all ite-expressions in all hypotheses as well as the goal.
split_ifs with h₁ h₂ h₃ overrides the default names for the hypotheses.