[InstSimplify] remove redundant calls to 'isImplied'; NFCI

We already call the more general isImpliedCondition() (which calls
isImpliedTrueByMatchingCmp() internally) from simplifyAndInst()
and simplifyOrInst().

There was a difference visible with this change on a vector test
before a925bef7, but I can't find any gaps now.