[ValueTracking] Remove untested / unreachable code, NFC

Right now isTruePredicate is only ever called with Pred == ICMP_SLE or
ICMP_ULE, and the ICMP_SLT and ICMP_ULT cases are dead.  This change
removes the untested dead code so that the function is not misleading.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252676 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed