AMDGPU: Use HasFP8ConversionInsts appropriately, NFC (#82433)

The corresponding fp8 conversion instructions are available for a
subtarget when and only when the subtarget "HasFP8ConversionInsts". We
should not assume all the future subtargets (gfx12+) have
FP8ConversionInsts.
  In this patch, we use OtherPredicates to carry HasFP8ConversionInsts
feature. This is because SubtargetPredicate is not copied from pseudos
to reals for DPP16 and DPP6. To avoid overriding OtherPredicates in a
few places, we use the newly introduced True16Predicate to hold
UseRealTrue16Insts instead.
 This work repalces the inadvertently closed pull request:
https://github.com/llvm/llvm-project/pull/82024