New exhaustiveness checker breaks T5642

The new exhaustiveness checker drastically increases compile time of the T5642 testcase. From the profile it appears that a great deal of time is being spent evaluating Check.mkPmId.occname,

COST CENTRE                 MODULE       %time %alloc

mkPmId.occname              Check         73.7   16.9
mkOneConFull                Check          3.4   10.2
deSugar                     HscMain        2.8   14.8
mkOneConFull.arguments      Check          2.5    5.5
pmTraverse                  Check          1.6    0.8
mkOneConFull.subst1         Check          1.5    5.9
wrapK.go                    Check          1.5    5.9
cMatcher                    Check          1.2    2.7
canEvVar                    TcCanonical    1.0    3.4

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	gkaracha
Operating system
Architecture