Make ConstPropagator only visit basic blocks when visiting body
Make ConstPropagator only visit basic blocks when visiting body

Avoid exercising unevaluated constants and go straight to visit the
blocks.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust