Add MCS jitflags support for the new GetLikelyClass PGO record type

[AndyAyersMS]

Records of this type are created when class profile histograms in dynamic PGO
data are summarized by the static PGO tooling.

These records can appear in both prejit and jit schemas when the static PGO data is
passed back to the jit.

[AndyAyersMS]

@BruceForstall PTAL
cc @dotnet/jit-contrib

You can see some of these in the recent asp.net collection:

aspnet.run.windows.x64.checked.mch

Grouped Flag Appearances (41826 contexts)

bits                count  percent  parsed
0000000048820000      167    0.40%  FEATURE_SIMD SKIP_VERIFICATION IL_STUB BBOPT
0000000248820000      127    0.30%  FEATURE_SIMD SKIP_VERIFICATION IL_STUB BBOPT PUBLISH_SECRET_PARAM
0000006248820000        1    0.00%  FEATURE_SIMD SKIP_VERIFICATION IL_STUB BBOPT PUBLISH_SECRET_PARAM REVERSE_PINVOKE TRACK_TRANSITIONS
0000008000820010     5003   11.96%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION TIER0
0000008020820010    18131   43.35%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBINSTR TIER0
0000000040820010     3808    9.10%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT
c000000040820010        9    0.02%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT HAS_PGO HAS_EDGE_PROFILE
0000010040820010     5008   11.97%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT TIER1
c000010040820010     6819   16.30%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT TIER1 HAS_PGO HAS_EDGE_PROFILE
d000010040820010      447    1.07%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT TIER1 HAS_PGO HAS_EDGE_PROFILE HAS_LIKELY_CLASS
e000010040820010     2173    5.20%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT TIER1 HAS_PGO HAS_EDGE_PROFILE HAS_CLASS_PROFILE
0000008080820010        6    0.01%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION FRAMED TIER0
00000080a0820010       60    0.14%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBINSTR FRAMED TIER0
00000000c0820010        6    0.01%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT FRAMED
00000100c0820010       26    0.06%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT FRAMED TIER1
c0000100c0820010       29    0.07%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT FRAMED TIER1 HAS_PGO HAS_EDGE_PROFILE
d0000100c0820010        3    0.01%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT FRAMED TIER1 HAS_PGO HAS_EDGE_PROFILE HAS_LIKELY_CLASS
e0000100c0820010        2    0.00%  DEBUG_INFO FEATURE_SIMD SKIP_VERIFICATION BBOPT FRAMED TIER1 HAS_PGO HAS_EDGE_PROFILE HAS_CLASS_PROFILE
0000000000820030        1    0.00%  DEBUG_INFO MIN_OPT FEATURE_SIMD SKIP_VERIFICATION

Individual Flag Appearances

   41531   99.29%  DEBUG_INFO
       1    0.00%  MIN_OPT
   41826  100.00%  FEATURE_SIMD
   41826  100.00%  SKIP_VERIFICATION
     295    0.71%  IL_STUB
   18191   43.49%  BBINSTR
   18625   44.53%  BBOPT
     132    0.32%  FRAMED
     128    0.31%  PUBLISH_SECRET_PARAM
       1    0.00%  REVERSE_PINVOKE
       1    0.00%  TRACK_TRANSITIONS
   23200   55.47%  TIER0
   14507   34.68%  TIER1
     450    1.08%  HAS_LIKELY_CLASS
    2175    5.20%  HAS_CLASS_PROFILE
    9482   22.67%  HAS_EDGE_PROFILE
    9482   22.67%  HAS_PGO

[AndyAyersMS]

This change doesn't impact runtime behavior, so arm test failures in System.Tests.GCTests.LatencyRoundtrips are caused by something else, perhaps #51547 ?