Don't run const propagation on items with inconsistent bounds

Using `#![feature(trivial_bounds)]`, it's possible to write functions
with unsatisfiable 'where' clauses, making them uncallable. However, the
user can act as if these 'where' clauses are true inside the body of the
function, leading to code that would normally be impossible to write.

Since const propgation can run even without any user-written calls to a
function, we need to explcitly check for these uncallable functions.

src/test/ui/trivial-bounds/trivial-bounds-inconsistent-associated-functions.rs

@@ -1,4 +1,8 @@
 // run-pass
+// Force mir to be emitted, to ensure that const
+// propagation doesn't ICE on a function
+// with an 'impossible' body. See issue #67696
+// compile-flags: --emit=mir,link
 // Inconsistent bounds with trait implementations
 
 #![feature(trivial_bounds)]

src/test/ui/trivial-bounds/trivial-bounds-inconsistent-associated-functions.stderr

@@ -0,0 +1,2 @@
+warning: due to multiple output types requested, the explicitly specified output file name will be adapted for each output type
+