[mlir][sparse] replace stack-based access pattern with dyn-alloc

[mlir][sparse] replace stack-based access pattern with dyn-alloc

Rationale:
Allocating the temporary buffers for access pattern expansion on the stack
(using alloca) is a bit too agressive, since it easily runs out of stack space
for large enveloping tensor dimensions. This revision changes the dynamic
allocation of these buffers with explicit alloc/dealloc pairs.

Reviewed By: bixia, wrengr

Differential Revision: https://reviews.llvm.org/D123253