rustc_passes/dead: avoid using TypeckTables::empty for MarkSymbolVisitor.
rustc_passes/dead: avoid using TypeckTables::empty for MarkSymbolVisitor.

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