[X86] Don't use NOPL when the assembler is passed an empty CPU string. Update tests to force a CPU with NOPL

Empty string should be equivalent to "generic" which doesn't allow NOPL. Force tests to use specificy 'pentiumpro' to guarantee NOPL.

Fixes PR35686

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321026 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed