#1025 Deal with side-channels in f8(l) port

[Philipp Klaus Krause]

Now that work on [feature-requests:#1006] has started, we should also look into the sf8(l)-specific aspects. We then need fixes or documentation. A first view shows:

• There are side-channels in codegen for comparison, shifts (right operand leaked), reads-from signed bit-fields, signed casts.
• There are no side-channels introduced in peephole optimizer rules
• There are side-channels in support routines.