Use prepareBinding in tryCastWorkerWrapperwip/T21144

As #21144 showed, tryCastWorkerWrapper was calling prepareRhs, and
then unconditionally floating the bindings, without the checks of
doFloatFromRhs.   That led to floating an unlifted binding into
a Rec group.

This patch refactors prepareBinding to make these checks,
and do them uniformly across all calls.  A nice improvement.

Other changes
* Instead of passing around a RecFlag and a TopLevelFlag; and sometimes
  a (Maybe SimplCont) for join points, define a new Simplifier-specific
  data type BindContext:
      data BindContext = BC_Let  TopLevelFlag RecFlag
                       | BC_Join SimplCont
  and use it consistently.

* Kill off completeNonRecX by inlining it.  It was only called in
  one place.

* Add a wrapper simplImpRules for simplRules.

Compile time on T9630 drops by 4.7%; little else changes.

Metric Decrease:
    T9630

0 files changed, 0 insertions, 0 deletions