Create and apply bitmasks for truncating casts.

Create and apply bitmasks for truncating casts.

When folding a cast, we were not applying the bitmask if we reached
a VARYING result.
We were also not creating a bitmask to represent the lower bits of a
truncating cast in op1_range.  So GORI was losing bits.

	PR tree-optimization/118254
	PR tree-optimization/114331
	gcc/
	* range-op.cc (operator_cast::fold_range): When VARYING is
	reached, update the bitmask if we reach VARYING.
	(operator_cast::op1_range): For truncating casts, create a
	bitmask bit in LHS.

	gcc/testsuite/
	* gcc.dg/pr114331.c: New.
	* gcc.dg/pr118254.c: New.