Fix multiple issues in LSRA for TYP_DOUBLE:

Fix multiple issues in LSRA for TYP_DOUBLE:
- updateAssignedInterval() needs to handle the case where the previously assigned interval was TYP_DOUBLE.
- Similarly, at block boundaries, if we are setting the register for a double interval, and the assigned interval is either null or is TYP_FLOAT ,we also need to unassign the other half of the register.
- LSRA only considers the valid double registers when allocating. When setting the candidates for the source of a return, it should only set the bit for the valid double register.

Fix #13751