[IROutliner] Deduplicating functions that only require inputs.

Extracted regions can have both inputs and outputs.  In addition, the
CodeExtractor removes inputs that are only used in llvm.assumes, and
sunken allocas (values are used entirely in the extracted region as
denoted by lifetime intrinsics).  We also cannot combine sections that
have different constants in the same structural location, and these
constants will have to elevated to argument. This patch deduplicates
extracted functions that only have inputs and non of the special cases.

We test that correctly deduplicate in:
test/Transforms/IROutliner/outlining-same-globals.ll
test/Transforms/IROutliner/outlining-same-constants.ll
test/Transforms/IROutliner/outlining-different-structure.ll

Reviewers: jroelofs, paquette

Differential Revision: https://reviews.llvm.org/D86978