Better IL output for property/field initializers

F#:
```fsharp
open System.Runtime.CompilerServices

type Test =

[]
val mutable X : int

new() = { }

[]
let test() =
Test(X = 1)

[]
let main _ =
0
```

Currently, F# compiler produces this IL:
```
.method public static class Program/Test
test() cil managed noinlining
{
// Code size 15 (0xf)
.maxstack 4
.locals init (class Program/Test V_0)
IL_0000: newobj instance void Program/Test::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: ldc.i4.1
IL_0008: stfld int32 Program/Test::X
IL_000d: ldloc.0
IL_000e: ret
} // end of method Program::test
```

However, the IL could be simplified to:
```
.method public static class Program/Test
test() cil managed noinlining
{
.maxstack 4
IL_0xxx: newobj instance void Program/Test::.ctor()
IL_0xxx: dup
IL_0xxx: ldc.i4.1
IL_0xxx: stfld int32 Program/Test::X
IL_0xxx: ret
}
```
No locals are produced and we have less instructions emitted.

@kerams - pinging you as I know you are interested in this stuff :)

I have some ideas on how to tackle this, but there may be better approaches that I'm not aware of.