[ValueTracking,VectorCombine] Allow passing DT to computeConstantRange.

isValidAssumeForContext can provide better results with access to the
dominator tree in some cases. This patch adjusts computeConstantRange to
allow passing through a dominator tree.

The use VectorCombine is updated to pass through the DT to enable
additional scalarization.

Note that similar APIs like computeKnownBits already accept optional dominator
tree arguments.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D110175