[mlir][bufferize] lower allocation alignment from 128 to 64 bytes

While it is unlikely to matter in practice, there is no reason
for this value to be larger than it should be. 64 bytes is the
size of a cache line in most machines, and we can fit a full
512-bit vector in it.

Reviewed By: springerm
Differential Revision: https://reviews.llvm.org/D139434