Annotation of SIMD loops

Use 'mark' nodes annotate a SIMD loop during ScheduleTransformation and skip
parallelism checks.

The buildbot shows the following compile/execution time changes:

  Compile time:
    Improvements    Δ     Previous  Current  σ
    …/gesummv      -6.06% 0.2640    0.2480   0.0055
    …/gemver       -4.46% 0.4480    0.4280   0.0044
    …/covariance   -4.31% 0.8360    0.8000   0.0065
    …/adi          -3.23% 0.9920    0.9600   0.0065
    …/doitgen      -2.53% 0.9480    0.9240   0.0090
    …/3mm          -2.33% 1.0320    1.0080   0.0087

  Execution time:
    Regressions     Δ     Previous  Current  σ
    …/viterbi       1.70% 5.1840    5.2720   0.0074
    …/smallpt       1.06% 12.4920   12.6240  0.0040

Reviewed-by: Tobias Grosser <tobias@grosser.es>

Differential Revision: http://reviews.llvm.org/D14491

llvm-svn: 261620