add and document regex support for FileCheck. You can now do stuff like:

add and document regex support for FileCheck.  You can now do stuff like:

; CHECK: movl {{%e[a-z][xi]}}, %eax

or whatever.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82717 91177308-0d34-0410-b5e6-96231b3b80d8