[FIRRTL] Fix pre-order walk in IMCP

IMCP needs to use a post-order walk as it is deleting operations (and the
operations are not skipped after erasure).  This should fix observed
failures in nightly CI [[1]].

[1]: 46014cd#commitcomment-146717821

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Author: seldridge

lib/Dialect/FIRRTL/Transforms/IMConstProp.cpp

@@ -1016,7 +1016,7 @@ void IMConstPropPass::rewriteModuleBody(FModuleOp module) {
   //
   // TODO: Handle WhenOps correctly.
   bool aboveCursor = false;
-  module.walk<mlir::WalkOrder::PreOrder, mlir::ReverseIterator>(
+  module.walk<mlir::WalkOrder::PostOrder, mlir::ReverseIterator>(
       [&](Operation *op) {
         auto dropIfDead = [&](Operation *op, const Twine &debugPrefix) {
           if (op->use_empty() &&