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[ValueTracking][ConstantRange] Distinguish low/high always overflow

Authored by nikic on May 28 2019, 11:08 AM.

Description

[ValueTracking][ConstantRange] Distinguish low/high always overflow

In order to fold an always overflowing signed saturating add/sub,
we need to know in which direction the always overflow occurs.
This patch splits up AlwaysOverflows into AlwaysOverflowsLow and
AlwaysOverflowsHigh to pass through this information (but it is
not used yet).

Differential Revision: https://reviews.llvm.org/D62463

llvm-svn: 361858

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