This MR fixes #24251.

See Note [Case-to-let for strictly-used binders]
in GHC.Core.Opt.Simplify.Iteration, plus #24251, for
lots of discussion.

Final Nofib changes over 0.1%:
+-----------------------------------------
|        imaginary/digits-of-e2    -2.16%
|                imaginary/rfib    -0.15%
|                    real/fluid    -0.10%
|                   real/gamteb    -1.47%
|                       real/gg    -0.20%
|                 real/maillist    +0.19%
|                      real/pic    -0.23%
|                      real/scs    -0.43%
|               shootout/n-body    -0.41%
|        shootout/spectral-norm    -0.12%
+========================================
|                     geom mean    -0.05%

Pleasingly, overall executable size is down by just over 1%.

Compile times (in perf/compiler) wobble around a bit +/- 0.5%, but the
geometric mean is -0.1% which seems good.