Allow writing an infix expression with not the same oprand type

**I propose we** allow writing an infix expression with not the same oprand type

(I do not know how to exactly explain it but it is something like this:
```fsharp
let inline (<<) a x =
(^t: (member Append: _ -> 't) (a, x))

let s = System.Text.StringBuilder()
s << 1 << 2 << 'a'
```

**The existing way of approaching this problem in F# is** ...

Seperate the line by the oprand types

```fsharp
let inline (<<) a x =
(^t: (member Append: _ -> 't) (a, x))

let s = System.Text.StringBuilder()
s << 1 << 2
s << 'a'
```

## Pros and Cons

**The advantages of making this adjustment to F# are** make the language consistent

**The disadvantages of making this adjustment to F# are** no

## Extra information

**Estimated cost (XS, S, M, L, XL, XXL):** Maybe S?

**Related suggestions:** (put links to related suggestions here)

## Affidavit (please submit!)

Please tick these items by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [StackOverflow](http://stackoverflow.com)) and I have searched StackOverflow for discussions of this issue
* [x] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to [the compiler and tooling repository](https://github.com/dotnet/fsharp)
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Start by examining how F# infix operators and member constraints are type-checked, comparing the single-type and mixed-operand examples in the issue. The proposal does not name implementation files or tests; done would require a settled language-design decision plus specified compiler behavior and corresponding tests.

Written by the indexing model from the issue text.