[flang] Rewrite "1*j" to "(j)", not "j", when j is a variable

Expression folding currently unconditionally rewrites "1*j"
to "j", which is wrong when "j" is a variable, as it transforms
an expression into a variable and can lead to incorrect associations
in contexts like an actual argument or an ASSOCIATE selector.
Transform "1*j" to a parenthesized "(j)" when "j" is a variable.
Fixes LLVM bug https://github.com/llvm/llvm-project/issues/63259.

Differential Revision: https://reviews.llvm.org/D153457