[GlobalISel] Translate <1 x N> getelementptrs to scalar G_PTR_ADDs

In `IRTranslator::translateGetElementPtr`, when we run into a vector gep with
some scalar operands, we try to normalize those operands using
`buildSplatVector`.

This is fine except for when the getelementptr has a <1 x N> type. In that case
it is treated as a scalar. If we run into one of these then every call to

```
// With VectorWidth = 1
LLT::fixed_vector(VectorWidth, PtrTy)
```

will assert.

Here's an example (equivalent to the added testcase):
https://godbolt.org/z/hGsTnMYdW

To get around this, this patch adds a variable, `WantSplatVector`, which
is true when our vector type ought to actually be represented using a vector.
When it's false, we'll translate as a scalar. This checks if `VectorWidth > 1`.

This fixes this bug:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=35496

Differential Revision: https://reviews.llvm.org/D105316