Teach transposition not to emit runWriters with trivial accumulators.
Teach transposition not to emit runWriters with trivial accumulators.

The theory is that this micro-optimization will improve compile times
by reducing the size of the IR that needs to be processed after
transposition.

Indeed, on a test example (not committed) of differentiating through
the log-density of a 1-D multivariate Gaussian, this
- reduces allocations by 2.5%
- reduces maximum memory residency by 10%
- seems to improve end-to-end execution time by 10% (subject to
  the usual measurement noise)

Co-authored-by: Dougal Maclaurin <0bb64e81bea11b43bfc7c06c1607c3af64f31e6d@google.com>

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