[DebugInfo][InstrRef] "final final" test cleanups for x86 tests

Two "totally definitely the last ones" instruction referencing test
updates:

 * fp-stack.ll: this test targets i686, and so it won't be getting
   instruction referencing, or at least not right now,
 * X86/live-debug-values.ll: instruction referencing will produce entry
   values in this test, add check lines to account for this. It's not clear
   what the test is supposed to be testing anyway, but the entry values
   appear to be correct.

Differential Revision: https://reviews.llvm.org/D114626