These changes fix missed cases where type-constrained values in if
tests and instance? tests were substituted in the left (consequent)
side of the if body computations, but not in the left side of the if
merge.

* sources/dfmc/optimization/assignment.dylan
  (maybe-rename-temporaries-in-conditional): Substitute the
   type-constrained value when the <if> test value appears in the left
   side of the <if-merge>.

* sources/dfmc/optimization/calls.dylan
  (do-optimize-instance?-user): Substitute the type-constrained value
   when the <if> instance? test value appears in the left side of the
   <if-merge>.

* documentation/source/release-notes/2024.2.rst: Add entry to Compiler
  section.