[X86] Simplify some code in combineTruncatedArithmetic. NFC
We haven't promoted AND/OR/XOR to vXi64 types for a while. So
there's no reason to use isOperationLegalOrPromote. So we can
just use isOperationLegal by merging with ADD handling.
craig.topper on May 3 2020, 11:53 PM.Authored by