[SVE][CodeGen] Legalisation of INSERT_VECTOR_ELT for scalable vectors

When the result type of insertelement needs to be split,
SplitVecRes_INSERT_VECTOR_ELT will try to store the vector to a
stack temporary, store the element at the location of the stack
temporary plus the index, and reload the Lo/Hi parts.

This patch does the following to ensure this works for scalable vectors:
 - Sets the StackID with getStackIDForScalableVectors() in CreateStackTemporary
 - Adds an IsScalable flag to getMemBasePlusOffset() and scales the
    offset by VScale when this is true
 - Ensures the immediate is clamped correctly by clampDynamicVectorIndex
    so that we don't try to use an out of range index

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D84874