Shrink `LiveNode`.

Shrink `LiveNode`.

`Liveness::users` is a vector that is occasionally enormous. For
example, doing a "clean incremental" check build of `inflate`, there is
one instance that represents 5,499 live nodes and 1087 vars, which
requires 5,977,413 entries. At 24 bytes per entry, that is 143MB.

This patch changes LiveNode from a usize to a u32. On 64-bit machines
that halves the size of these entries, significantly reducing peak
memory usage and memory traffic, and speeding up "clean incremental"
builds of `inflate` by about 10%.