Check if operand is an index expr in simplify_function_application()

[danpoe]

Previously the same expression was accidentally checked twice.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: aee5e3d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114770302

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@d841075). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #4766   +/-   ##
==========================================
  Coverage           ?   68.51%           
==========================================
  Files              ?     1275           
  Lines              ?   105032           
  Branches           ?        0           
==========================================
  Hits               ?    71958           
  Misses             ?    33074           
  Partials           ?        0

Impacted Files	Coverage Δ
src/util/simplify_expr.cpp	80.69% <ø> (ø)

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[smowton]

@peterschrammel can override the apparent AWS outage

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0a72c1e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114917888