[InstCombine] Bail out of load-store forwarding for scalable vector types

This patch fixes an invalid TypeSize->uint64_t implicit conversion in
FoldReinterpretLoadFromConst. If the size of the constant is scalable
we bail out of the optimisation for now.

Tests added here:

  Transforms/InstCombine/load-store-forward.ll

Differential Revision: https://reviews.llvm.org/D120240