[FuzzMutate] Fix getWeight of InstDeleterIRStrategy

The comment states the following, for calculating the Line variable:

> Draw a line starting from when we only have 1k left and increasing
> linearly to double the current weight.

However, the value was not calculated as described. Instead, it would
result in a negative value, which resulted in the function always
returning 0 afterwards.

```
// Invariant: CurrentSize <= MaxSize - 200
// Invariant: CurrentWeight >= 0
int Line = (-2 * CurrentWeight) * (MaxSize - CurrentSize + 1000);
// {Line <= 0}
```

This commit fixes the issue and linearly interpolates as described.

Patch by Loris Reiff. Thanks!

Differential Revision: https://reviews.llvm.org/D96207