[DAGCombiner] Teach MatchContextClass classes to use TargetLowering::isOperationLegalOrCustom().

Some of TargetLowering functions needed opcodes are often used in DAGCombiner.
The patch make those MatchContextClass classes have TargetLowering members and
pass specific opcodes for those TargetLowering functions.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D144075