[mlir] Fix Python bindings tests failure in Debug mode after D98474

Add extra `type.isa<FloatType>()` check to  `FloatAttr::get(Type, double)` method.
Otherwise it tries to call `type.cast<FloatType>()`, which fails with assertion in Debug mode.

The `!type.isa<FloatType>()` case just redirercts the call to `FloatAttr::get(Type, APFloat)`,
which will perform the actual check and emit appropriate error.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D98764