tv/fltcalc: Create fp_values with long double info and convert later.

fc_val_from_ieee754 simply does not work correctly for denormalized
input values if the desired result is smaller than a long double.
However, converting to an fp_value using the mantissa/characteristic
information of a long double and later using fc_cast to convert to
the desired target format works.
Rewrite and clean up SSA destruction.
Add support for implementing cycles with copies and free register.
Revert "Clear corresponding irg property when creating Bads."

This reverts commit 3d1d2c43d16a6166989a9a5f69a266cafbe1f723.

Creating a Bad node does not imply that it has users or is reachable
within an irg. While clearing the property is certainly correct as a
conservative approximation, it should not be necessary.
sparc: Remember call type attribute in sparc_Call node.
Do not coalesce spillslots if graph contains a call that returns twice.

This works around a problem with modeling control flow for calls to
setjmp and similar functions.
refactoring of returns_twice=>no_coalescing rule

This mainly adds comments explaining the situation and avoids an extra
graph walk.