[WhileToFor] If not moved into after region

Nobody has claimed this yet.

As an example:
```c
unsigned hash_thingy(unsigned v) {
for (int i = 0; i < 3; ++i) {
v = (v ^ 234567) ^ (v >> 16);
}
for (int i = 0; i < 3; ++i) {
v = (v ^ 12345) ^ (v >> 16);
}
return v;
}
```

lowers to:
```mlir
func @hash_thingy(%arg0: i32) -> i32 attributes {llvm.linkage = #llvm.linkage} {
%c0 = arith.constant 0 : index
%c0_i32 = arith.constant 0 : i32
%c3_i32 = arith.constant 3 : i32
%c234567_i32 = arith.constant 234567 : i32
%c16_i32 = arith.constant 16 : i32
%c1_i32 = arith.constant 1 : i32
%c3 = arith.constant 3 : index
%c1 = arith.constant 1 : index
%0:3 = scf.while (%arg1 = %c0_i32, %arg2 = %arg0) : (i32, i32) -> (i32, i32, i32) {
%2 = arith.cmpi slt, %arg1, %c3_i32 : i32
%3:3 = scf.if %2 -> (i32, i32, i32) {
%4 = arith.xori %arg2, %c234567_i32 : i32
%5 = arith.shrui %arg2, %c16_i32 : i32
%6 = arith.xori %4, %5 : i32
%7 = arith.addi %arg1, %c1_i32 : i32
%8 = llvm.mlir.undef : i32
scf.yield %7, %6, %8 : i32, i32, i32
} else {
scf.yield %arg1, %arg2, %arg2 : i32, i32, i32
}
scf.condition(%2) %3#0, %3#1, %3#2 : i32, i32, i32
} do {
^bb0(%arg1: i32, %arg2: i32, %arg3: i32): // no predecessors
scf.yield %arg1, %arg2 : i32, i32
}
%1:2 = scf.for %arg1 = %c0 to %c3 step %c1 iter_args(%arg2 = %0#2, %arg3 = %0#2) -> (i32, i32) {
%2 = arith.xori %arg2, %c234567_i32 : i32
%3 = arith.shrui %arg2, %c16_i32 : i32
%4 = arith.xori %2, %3 : i32
scf.yield %4, %4 : i32, i32
}
return %1#1 : i32
}
```

The second loop makes good sense, but the first one is a bit strange due to being lowered to a while loop as well as the existence of the `llvm.mlir.undef` operation.

The issue names the WhileToFor lowering but no source file or test. Reproduce the hash_thingy example and inspect the generated MLIR, then trace the first loop through the WhileToFor entry point. Done means an agreed lowering avoids the reported unnecessary scf.while and llvm.mlir.undef behavior, with a regression test if the project has one.

Written by the indexing model from the issue text.