A FileCheck test (e.g. codegen) with dead annotations should error

[RalfJung]

The following line is accidentally not doing anything:

rust/tests/codegen/float/f16.rs

Line 147 in 57d5f86

// other-LABEL: half @f16_as_self(

There is no other revision in that file, so this is a NOP. And apparently that doesn't trigger any sort of error...

Cc @fmease @jieyouxu

[fmease]

cc #134510 ig