Restore post-order simplification of byte_extract(c ? a : b, ...)

[tautschnig]

With 5d29491 it was assumed that this step was fully taken care of by pre-order simplification passes. Other post-order simplification rules, however, may newly introduce such expressions, making it necessary that we also do this in post-order traversal.

Absence of this rule leads to doubling of memory consumption when trying to prove aws_cryptosdk_keyring_trace_copy_all in aws-encrption-sdk-c.

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[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (b836b4f) 77.98% compared to head (56d171a) 77.90%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8005      +/-   ##
===========================================
- Coverage    77.98%   77.90%   -0.09%     
===========================================
  Files         1701     1701              
  Lines       196502   196509       +7     
===========================================
- Hits        153245   153081     -164     
- Misses       43257    43428     +171     

Files	Coverage Δ
src/util/simplify_expr.cpp	62.77% <0.00%> (-9.20%)	⬇️

... and 93 files with indirect coverage changes

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