[SVE] Fix TypeSize->uint64_t implicit conversion in visitAlloca()

Fixes a crash ('Invalid size request on a scalable vector') in visitAlloca()
when we call this function for a scalable alloca instruction, caused
by the implicit conversion of TySize to uint64_t.
This patch changes TySize to a TypeSize as returned by getTypeAllocSize()
and ensures the allocation size is multiplied by vscale for scalable vectors.

Reviewed By: sdesmalen, david-arm

Differential Revision: https://reviews.llvm.org/D118372