Teach the DAG combiner to turn chains of FADDs (x+x+x+x+...) into FMULs by...

Teach the DAG combiner to turn chains of FADDs (x+x+x+x+...) into FMULs by constants.  This is only enabled in unsafe FP math mode, since it does not preserve rounding effects for all such constants.

llvm-svn: 162956