while ... do: inconsistent handling of whitespace / indentation

`while ... do` expressions handle whitespace / indentation in a manner that is not consistent with `if ... then ... else` expressions.
This is problematic when the test expression is lengthy and would be more clearly written using multiple lines, along the lines of:

``` fsharp
while
x > 10
&&
x < 20
do ...
```

**Repro steps**

Provide the steps required to reproduce the problem:

``` fsharp
let mutable x = 0
while
x < 10
do
x <- x + 1
```

**Expected behavior**

I would expect this to be valid syntax, in a fashion similar to

``` fsharp
if
x < 10
then
// do something
else
// do something
```

**Actual behavior**

`error FS0010: Incomplete structured construct at or before this point in expression`
`Missing 'do' in 'while' expression. Expected 'while do '.`

**Known workarounds**

Any indentation of `do` resolves the issue:

``` fsharp
let mutable x = 0
while
x < 10
do
x <- x + 1
```