Don't optimize coercions with -O0

During the call today Simon and I discussed the possibility of omitting coercion optimization from the desugarer's simple optimization (simpleOptPgm) when compiling with -O0. The justification here was to perhaps speed-up unoptimized compilation times for programs like that of #12506, which has very large coercions (presumably due to #8095).

The rationale was that while this first pass of coercion simplification during desugaring may reduce the size of some of these coercions, we only walk over them once more during code generation, so there's little benefit given the cost.

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Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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