rustbuild: rustc_codegen_llvm is recompiled when any rustc crate is changed

[arielb1]

STR

$ ../x.py test --incremental src/test/{compile-fail,ui,run-pass}
Updating only changed submodules
Submodules updated in 0.03 seconds
    Finished dev [unoptimized] target(s) in 0.24s                                                                                                                                                  
Building stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.25s                                                                                                                                                
Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 test artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.23s                                                                                                                                                
Copying stage0 test from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compiling rustc_typeck v0.0.0 (file:///home/ariel/Rust/rust-master/src/librustc_typeck)                                                                                                         
   Compiling rustc_save_analysis v0.0.0 (file:///home/ariel/Rust/rust-master/src/librustc_save_analysis)
   Compiling rustc_privacy v0.0.0 (file:///home/ariel/Rust/rust-master/src/librustc_privacy)
   Compiling rustc_driver v0.0.0 (file:///home/ariel/Rust/rust-master/src/librustc_driver)
   Compiling rustc-main v0.0.0 (file:///home/ariel/Rust/rust-master/src/rustc)
    Finished release [optimized] target(s) in 24.95s
Copying stage0 rustc from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 codegen artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu, llvm)
   Compiling libc v0.2.43                                                                                                                                                                          
   Compiling rustc-demangle v0.1.9
   Compiling cc v1.0.22
   Compiling rustc_codegen_llvm v0.0.0 (file:///home/ariel/Rust/rust-master/src/librustc_codegen_llvm)
   Compiling rustc_llvm v0.0.0 (file:///home/ariel/Rust/rust-master/src/librustc_llvm)
warning: In file included from /home/ariel/Rust/rust-master/src/llvm/include/llvm/CodeGen/TargetSubtargetInfo.h:22,
warning:                  from ../rustllvm/PassWrapper.cpp:29:
warning: /home/ariel/Rust/rust-master/src/llvm/include/llvm/CodeGen/SchedulerRegistry.h: In constructor ‘llvm::RegisterScheduler::RegisterScheduler(const char*, const char*, llvm::RegisterScheduler::FunctionPassCtor)’:
warning: /home/ariel/Rust/rust-master/src/llvm/include/llvm/CodeGen/SchedulerRegistry.h:40:52: warning: cast between incompatible function types from ‘llvm::RegisterScheduler::FunctionPassCtor’ {aka ‘llvm::ScheduleDAGSDNodes* (*)(llvm::SelectionDAGISel*, llvm::CodeGenOpt::Level)’} to ‘llvm::MachinePassCtor’ {aka ‘void* (*)()’} [-Wcast-function-type]
warning:    : MachinePassRegistryNode(N, D, (MachinePassCtor)C)
warning:                                                     ^
   Compiling num_cpus v1.8.0
    Finished release [optimized] target(s) in 1m 02s
Assembling stage1 compiler (x86_64-unknown-linux-gnu)
Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
...

This also occurs with keep-stage0 etc.

Expected result

Changing librustc_typeck requires only the reverse dependencies of librustc_typeck to be recompiled.

Actual result

librustc_codegen_llvm is also recompiled. As you can see, this takes twice as much as compiling the reverse dependencies of librustc_typeck alone, leading to compilation being 3x slower.