[mlir] Set insertion point of vector constant to the top of the vectorized loop body

When we vectorize a scalar constant, the vector constant is inserted before its
first user if the scalar constant is defined outside the loops to be vectorized.
It is possible that the vector constant does not dominate all its users. To fix
the problem, we find the innermost vectorized loop that encloses that first user
and insert the vector constant at the top of the loop body.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D106609