[ValueTracking] Use computeConstantRange() for unsigned add/sub overflow

Improve computeOverflowForUnsignedAdd/Sub in ValueTracking by
intersecting the computeConstantRange() result into the ConstantRange
created from computeKnownBits(). This allows us to detect some
additional never/always overflows conditions that can't be determined
from known bits.

This revision also adds basic handling for constants to
computeConstantRange(). Non-splat vectors will be handled in a followup.

The signed case will also be handled in a followup, as it needs some
more groundwork.

Differential Revision: https://reviews.llvm.org/D59386

llvm-svn: 356489

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