The index used to index into a structure (a scalar) used to have the
vector size of the resulting type. This changed recently to always be
1, so the field type size needs to be determined from the fields
themselves, which is also the type of the indexing result.

Change-Id: I2dc373d8d31b02c0f69879cb0a3feacd83d6e473
Reviewed-on: https://swiftshader-review.googlesource.com/8368Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>