[pseudo] Add a fast-path to GLR reduce when both pop and push are trivial

In general we split a reduce into pop/push, so concurrently-available reductions
can run in the correct order. The data structures for this are expensive.

When only one reduction is possible at a time, we need not do this: we can pop
and immediately push instead.
Strictly this is correct whenever we yield one concurrent PushSpec.

This patch recognizes a trivial but common subset of these cases:
 - there must be no pending pushes and only one head available to pop
 - the head must have only one reduction rule
 - the reduction path must be a straight line (no multiple parents)

On my machine this speeds up by 2.12 -> 2.30 MB/s = 8%

Differential Revision: https://reviews.llvm.org/D128299