[RISCV] Skip DAG combine for bitcast fabs/fneg (#115325)
Disable the DAG combine for bitcast fabs/fneg in case of the zdinx
extension.
The combine folds the fabs/fneg nodes in some cases. This might result
in suboptimal code if compiled with the zdinx extension. In case of the
zdinx extension, there is no need to load the double value from an x
register to an f register, so the combine can be skipped.
