[flang] Avoid code duplication in mixed expressions

Rather than represent the mixed real/complex subexpression x*(a,b)
as (x*a,x*b), use (x,0)*(a,b) to avoid a potential code duplication
in current lowering code.  Same for mixed division, and for mixed
integer*complex and integer/complex cases.

Differential Review: https://reviews.llvm.org/D115732