[X86] Rename tests for bypassing slow division (NFC)

For tests on bypassing slow division there's no need to be
Atom-specific. The patch renames all tests on division bypassing
and makes their names more consistent:

  atom-bypass-slow-division.ll -> bypass-slow-division-32.ll
  (tests verifying correctness of divl-to-divb bypassing)

  atom-bypass-slow-division-64.ll -> bypass-slow-division-64.ll
  (tests verifying correctness of divq-to-divl bypassing)

  slow-div.ll -> bypass-slow-division-tune.ll
  (tests verifying that bypassing is enabled only when appropriate)

Differential Revision: https://reviews.llvm.org/D28197

llvm-svn: 291802