Type annotations for instruction operands are omitted, e.g.
```
%3 = struct $S(%1, %2)
```
Operand types are redundant anyway and were only used for sanity checking in the SIL parser.
But: operand types _are_ printed if the definition of the operand value was not printed yet.
This happens:
* if the block with the definition appears after the block where the operand's instruction is located
* if a block or instruction is printed in isolation, e.g. in a debugger
The old behavior can be restored with `-Xllvm -sil-print-types`.
This option is added to many existing test files which check for operand types in their check-lines.
If a var has optional type, e.g.
```
var x: Int?
```
It will be implicitly initialized to `nil`. However, there's a second,
undocumented behavior: tuples of optional type, potentially nested infinitely,
will also be initialized, to tuples of nil.
So this var
```
var w: ((Int?, (), Int?), (Int?, Int?))
```
Will be default-initialized to
```
((nil, (), nil), (nil, nil))
```
We need to handle this inside getDefaultValueStringRepresentation,
otherwise we will crash while emitting partial modules.
Fixes one instance of rdar://51560190
