[X86] Modify BypassSlowDivision tests to match their new names (NFC)

- bypass-slow-division-32.ll:
  tests verifying correctness of divl-to-divb bypassing

- bypass-slow-division-64.ll:
  tests verifying correctness of divq-to-divl bypassing

- bypass-slow-division-tune.ll:
  tests verifying that bypassing is enabled only when appropriate

Differential Revision: https://reviews.llvm.org/D28551

llvm-svn: 291804