ValueTracking: Recognize fpclass clamping select patterns

Improve computeKnownFPClass select handling to cover the case where
the condition performs a class test. This allows us to recognize
no-nans in cases like:

  %not.nan = fcmp ord float %x, 0.0
  %select = select i1 %not.nan, float %x, float 0.0

Math library code has similar edge case filtering on the inputs and
final results.

https://reviews.llvm.org/D153089