While this resolves the regression from D73722 in llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll,
this now regresses llvm/test/Transforms/IndVarSimplify/elim-extend.ll @nestedIV test,
we no longer can perform that expansion within default budget of 4, but require budget of 6.
That regression is being addressed by D73777.
The basic idea here is simple.
Op0, Op1, Op2 ... | | | \--+--/ | | | \---+---/
I.e. given N operands, we will have N-1 operations,
so we have to add cost of an add (mul) for every Op processed,
except the first one, plus we need to recurse into *every* Op.
I'm guessing there's already canonicalization that ensures we won't
have 1 operand in scMulExpr, and no 0 in scAddExpr/scMulExpr.