[X86] Fix assertion failure caused by a wrong folding of vector shifts by immediate count.

This fixes a regression intruced by r198113.

Revision r198113 introduced an algorithm that tries to fold a vector shift
by immediate count into a build_vector if the input vector is a known vector
of constants.

However the algorithm only worked under the assumption that the input vector
type and the shift type are exactly the same.

This patch disables the folding of vector shift by immediate count if the
input vector type and the shift value type are not the same.