[LV] Prevent vectorization with unsupported element types.

This patch adds a TTI function, isElementTypeLegalForScalableVector, to query
whether it is possible to vectorize a given element type. This is called by
isLegalToVectorizeInstTypesForScalable to reject scalable vectorization if
any of the instruction types in the loop are unsupported, e.g:

  int foo(__int128_t* ptr, int N)
    #pragma clang loop vectorize_width(4, scalable)
    for (int i=0; i<N; ++i)
      ptr[i] = ptr[i] + 42;

This example currently crashes if we attempt to vectorize since i128 is not a
supported type for scalable vectorization.

Reviewed By: sdesmalen, david-arm

Differential Revision: https://reviews.llvm.org/D102253