David Majnemer cd4fbcd1bb InstSimplify: Simplify (X - (0 - Y)) if the second sub is NUW ...

If the NUW bit is set for 0 - Y, we know that all values for Y other
than 0 would produce a poison value.  This allows us to replace (0 - Y)
with 0 in the expression (X - (0 - Y)) which will ultimately leave us
with X.

This partially fixes PR20189.

llvm-svn: 214384

2014-07-31 04:49:18 +00:00

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