Fix duplicate name when lowering scf.if

[rikhuijzer]

Fixes a bug where variables are not automatically renamed when lowered from inside the region. This, for example, comes up when lowering

func.func @main() -> i64 {
  %x = arith.constant false
  scf.if %x {
    %0 = arith.constant 2 : i64
  } else {
    %0 = arith.constant 3 : i64
  }
  %1 = arith.constant 0 : i64
  return %1 : i64
}

to

func.func @main() -> i64 {
  %0 = arith.constant false
  cf.cond_br %0, ^bb1, ^bb2
^bb1: 
  %1 = arith.constant 2 : i64
  cf.br ^bb3
^bb2:
  %2 = arith.constant 3 : i64
  cf.br ^bb3
^bb3:
  %3 = arith.constant 0 : i64
  return %3 : i64
}

Notice that the second definition of %0 has to be renamed.

Now it makes sense why MLIR always renames variables. If you don't, any variable that is inlined and has a name that is the same as another variable needs to have a new name. But say you have %0, %1 and now you inline another %0. What name would you give it? %0_0? You cannot just call it %2 because, if I'm not mistaken, LLVM will then start to complain (fair enough) because %2 occurs before %1. Just ignoring names and giving everything a new ("fresh") name leads to much cleaner generated code.