[X86] Simplify patterns for avx512 vpcmp. NFC

This removes the commuted PatFrags that only existed to carry
an SDNodeXForm in its OperandTransform field. We know all the places
that need to use the commuted SDNodeXForm and there is one transform
shared by signed and unsigned compares. So just hardcode the
the SDNodeXForm where it is needed and use the non commuted PatFrag
in the pattern.

I think when I wrote this I thought the SDNodeXForm name had to
match what is in the PatFrag that is being used. But that's not
true. The OperandTransform is only used when the PatFrag is used
in an instruction pattern and not a separate Pat pattern. All
the commuted cases are Pat patterns.