Disable decimal floating point in frange.

Disable decimal floating point in frange.

As Jakub mentioned in the PR, because many numbers have multiple
possible representations, we can't reliably return true for singleton_p.
For that matter, we may not be capable of modeling them just yet.
Disabling them until someone with DFP knowledge can opine or extend
frange.

	PR middle-end/106831

gcc/ChangeLog:

	* value-range.h (frange::supports_p): Disable decimal floats.
	* range-op-float.cc (frange_drop_inf): Remove DECIMAL_FLOAT_MODE_P
	check.
	(frange_drop_ninf): Same.