Fix faulty substitutions in StgCse (#11532).
Fix faulty substitutions in StgCse (#11532).

`substBndr` should rename bindings which shadow existing ids.
However while it was renaming the bindings it was not adding proper substitutions
for renamed bindings.
Instead of adding a substitution of the form `old -> new` for renamed
bindings it mistakenly added `old -> old` if no replacement had taken
place while adding none if `old` had been renamed.

As a byproduct this should improve performance, as we no longer add
useless substitutions for unshadowed bindings.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc