Fix use counts for mutable struct SROA
PR #28478 moved the computation of the use counts before the finish call.
to fix #28444. However, the early parts of the finish call fixes up phi
node arguments, which fail to get counted if we look at use counts before
that fixup is performed. This causes #30594 where the only non-trivial use
is on the backedge of the phi and would thus incorrectly fail to get accounted
for. Fix that by taking the use count after phi fixup but before dce.
(cherry picked from commit f8f20453c4ca7a6aeb23a189a420dbff130679ce)
