[llvm] [KnownFPClass] Fix KnownFPClass::frem subnormal handling and improve deductions. (PR #219303)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

Also, with regards to `PPCDoubleDouble` (see also https://github.com/llvm/llvm-project/issues/217658), one will have to be careful when ruling out `subnormal` since `frem(1.0 + DBL_TRUE_MIN, 1.0) == DBL_TRUE_MIN == subnormal`, but I believe this PR should not have any `PPCDoubleDouble` bugs since we rule out `fcPosNormal` and `fcPosSubnormal` together instead of separately.


https://github.com/llvm/llvm-project/pull/219303