Support ExtVectorType conditional operator

Extension vectors now can be used in element-wise conditional selector.
For example:
```
R[i] = C[i]? A[i] : B[i]
```
This feature was previously only enabled in OpenCL C. Now it's also
available in C. Not that it has different behaviors than GNU vectors
(i.e. __vector_size__). Extension vectors selects on signdness of the
vector. GNU vectors on the other hand do normal bool conversions. Also,
this feature is not available in C++.

Differential Revision: https://reviews.llvm.org/D80574