Ensure InlineCasts does not inline complex Expressions (#2130)

Previously, InlineCasts could inline complex (ie. non-cast) Expressions
into other complex Expressions. Now it will only inline so long as there
no more than 1 complex Expression in the current nested Expression.

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
(cherry picked from commit b274b319)

# Conflicts:
#	src/main/scala/firrtl/transforms/InlineCasts.scala
#	src/test/scala/firrtlTests/InlineCastsSpec.scala