acc/pass: handle missing allocations

[mmcloughlin]

The variable allocator currently raises an assertion error if it encounters a
write to an operand without an allocation. Allocation is done like liveness
analysis: the instructions are processed in reverse and variables are
allocated as they are read, freed when they're written to. Therefore this
assertion would trigger if the operand is never read after it's written.

Therefore, this would only happen for a redundant chain which computes a
value it never needs. Clearly this isn't ideal, but it does seem to happen for
some algorithms. It seems best that the allocator doesn't crash in these
cases.

This PR updates the allocation logic to handle this case. In practice if this
happens the variable would be immediately allocated and freed.

Updates #129 #133 #136

[codecov-commenter]

Codecov Report

Merging #137 (0667815) into master (33f7a03) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
+ Coverage   73.76%   74.00%   +0.23%     
==========================================
  Files          36       36              
  Lines        2169     2177       +8     
==========================================
+ Hits         1600     1611      +11     
+ Misses        494      492       -2     
+ Partials       75       74       -1     

Flag	Coverage Δ
unittests	74.00% <100.00%> (+0.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
acc/pass/alloc.go	95.58% <100.00%> (+5.58%)	⬆️

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