[IROutliner] Merging identical output blocks for extracted functions.

Many of the sets of output stores will be the same. When a block is
created, we check if there is an output block with the same set of store
instructions. If there is, we map the output block of the region back
to the block, so that the extra argument controlling the switch
statement can be set to the appropriate block value.

Tests:
- llvm/test/Transforms/IROutliner/outlining-same-output-blocks.ll

Reviewers: jroelofs, paquette

Differential Revision: https://reviews.llvm.org/D87298