This patch ensures we taken into account COMPLETE pragmas when we
compute whether a pattern is irrefutable. In particular, if a pattern
synonym is the sole member of a COMPLETE pragma (without a result TyCon),
then we consider a pattern match on that pattern synonym to be irrefutable.

This affects the desugaring of do blocks, as it ensures we don't use
a "fail" operation.

Fixes #15681 #16618 #22004