[RegisterClassInfo] Use SmallVector::assign instead of resize to make sure we erase...

[RegisterClassInfo] Use SmallVector::assign instead of resize to make sure we erase previous contents from all entries of the vector.

resize only writes to elements that get added. Any elements that
already existed maintain their previous value. In this case we're
trying to erase cached information so we should use assign which
will write to every element.

Found while trying to add new tests to an existing X86 test and
 noticed register allocation changing in other functions.