IAS: support .rep as an alias for .rept

The GNU assembler supports .rep as an alias for .rept.  This simply creates the
alias for it and introduces a test for both .rept and .rep.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198097 91177308-0d34-0410-b5e6-96231b3b80d8