InstCombine: Fold logic of fp_classes together

Move logical operators on pairs of llvm.is.fpclass on the same value
into the test mask of a single is_fpclass.

or (class x, mask0), (class x, mask1) -> class x, (mask0 | mask1)
and (class x, mask0), (class x, mask1) -> class x, (mask0 & mask1)
xor (class x, mask0), (class x, mask1) -> class x, (mask0 ^ mask1)

The and/or cases should appear frequently in the builtin math
libraries; haven't seen the xor case but handle it for completeness.