make if-conversion and archdep optimisations get their information directly from the backend without frontends needing special code to transfer these infos

[r27937]

[r27936]

[r27924]

[r27923]

[r27919]

[r27918]

[r27917]

[r27916]

[r27915]

[r27914]

[r27913]

sparc: No need for Minus, Not, Nop nodes. Implement with the real instructions Sub, XNor and zero register

[r27912]

[r27911]

[r27910]

[r27909]

[r27908]

[r27907]

[r27906]

[r27905]

[r27904]

[r27903]

[r27902]

[r27901]

[r27900]

The gain from using an extra Unreachable is rather low, plus the noreturn call must probably be keeped anyway, so revrt it again

[r27898]

Unreachable nodes mark unreachable control flow (after noreturn calls). They
"eat" the memory, saving the two keeps used before.

Works mostly yet (scheduler issues warning about not scheduled Unreachable's)

[r27894]

[r27892]

[r27891]

[r27890]

[r27887]

[r27886]

[r27885]

[r27884]

[r27883]

[r27882]

[r27881]

[r27879]

[r27878]

[r27877]

[r27875]