[Custom Descriptors] Optimize branching descriptor casts (#7727)

Update RemoveUnusedBrs to optimize br_on_cast_desc and
br_on_cast_desc_fail in addition to the existing branching casts.

Author: tlively

scripts/test/fuzzing.py

@@ -130,6 +130,7 @@
     'gto-desc.wast',
     'type-ssa-desc.wast',
     'abstract-type-refining-desc.wast',
+    'remove-unused-brs-desc.wast',
     # TODO: fix split_wast() on tricky escaping situations like a string ending
     #       in \\" (the " is not escaped - there is an escaped \ before it)
     'string-lifting-section.wast',