ComputeLineNumbers: delete SSE2 vectorization

Summary:
SSE2 vectorization was added in 2012, but it is 2018 now and I can't
observe any performance boost (testing clang -E [all Sema/* CodeGen/* with proper -I options]) with the existing _mm_movemask_epi8+countTrailingZeros or the following SSE4.2 (compiling with -msse4.2):

  __m128i C = _mm_setr_epi8('\r','\n',0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  _mm_cmpestri(C, 2, Chunk, 16, _SIDD_UBYTE_OPS | _SIDD_CMP_EQUAL_ANY | _SIDD_POSITIVE_POLARITY | _SIDD_LEAST_SIGNIFICANT)

Delete the vectorization to simplify the code.

Also simplify the code a bit and don't check the line ending sequence \n\r

Reviewers: bkramer, #clang

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55484

llvm-svn: 348777