introduced the SierraMask type that replaces the former CGF.CurrentMask;
transformed Sierra code gen to use SierraMasks;
added support for break/continue to loop-stmts and the if-stmt
fixed some cases where equal values have been computed multiple times;
fixed masking and branching for the Sierra-if
implemented support for break/continue in the llops and the if-stmt;
the if-stmt now merges the active lanes from the then and the else successor;
there still is a bug in the Sierra for stmt, probably the increment stmt is not
correctly masked
some renaming
introduced phi-nodes for the Continue block in the for-stmt to collect the
current-/continue-mask after a break/continue
implemented break and continue for sierra loops:
- CGF now keeps a stack of loop masks and a stack of continue-phi-nodes
- sierra continue causes a new incoming edge to the continue-phi
- sierra break and sierra continue now check whether all lanes are dead in the
  loop, and trigger a conditional branch to the break-/continue-target BB
implemented Sierra return
Merge branch 'Sierra-BreakContinue'