[mlir][emitc] Refactor brackets in expressions (#168267)
This patch is a minor NFC-intended refactoring to the way emitting
redundant parentheses is prevented.
The current implementation pushes and later pops a fake low precedence
into the precedence stack when emitting function calls. The new
implementation adds a boolean argument to `emitOperand()` that explicity
guarantees that the operand is being emitted between some kind of
brackets, exempting the method from enforcing correct evaluation order
w.r.t precedence and associativity up the expression tree.
