[IR] Use User::getHungOffOperands() in HungoffOperandTraits::op_begin/op_end(). NFC (#74744)

User::getOperandList has to check the HasHungOffUses bit in Value to
determine how the operand list is stored. If we're using
HungoffOperandTraits we can assume how it is stored without checking the
flag.

Noticed that the for loop in matchSimpleRecurrence was triggering loop
unswitch when built with clang due to specializing based on how the
operand list of the PHINode was stored.

This reduces the size of llc on my local Release+Asserts build by around
41K.