[mlir] Fix casting of leading unit dims for vector.insert

When dropping leading unit dims of vector.insert's operands and creating
a new vector.insert, its new position rank should be computed explicitly
in two steps: first based on the numbers of leading unit dims dropped
from the vector.insert's destination, then based on the numbers of
leading unit dims dropped from its source.

Reviewed By: pifon2a

Differential Revision: https://reviews.llvm.org/D147280