[IROutliner] Using canonical values to find corresponding values. (NFC)

D104143 introduced canonical value numbering between regions, which allows for the easy identification of items across a region, eliminating the need in the outliner to create parallel lists of instructions for each region, and replace output values in a less convoluted way.

Additionally, in a future commit, the output values will not necessarily be recorded values from the region itself, it could be a combination value where the actual value being output is a PHINode instead.  This new method allows us to handle the replacement of the output value to the stored value with the corresponding item in the same place for both normal output values, and PHINode outputs instead of handling the different types of outputs in different locations.

Reviewers: paquette, roelofs

Differential Revision: https://reviews.llvm.org/D108656