Change NYI to be a noway_assert if ALT_JIT is not defined

[BruceForstall]

We recently changed RyuJIT/x86 to be the default JIT. However, there
are still a few NYI that can happen, such as when enabling SIMD,
which is not enabled yet. However, there is no "fallback" JIT anymore.
So just treat all NYI as asserts.

[BruceForstall]

@dotnet/jit-contrib PTAL

[BruceForstall]

@mmitche @swgillespie my "x64 Release Priority 1 Build and Test" failed with:

C:\Program Files (x86)\MSBuild\14.0\bin\Microsoft.Common.CurrentVersion.targets(4412,5): error MSB3491: Could not write lines to file "D:\j\workspace\x64_release_w---0575cb46\bin\int\Managed\GC\Stress\Tests\DirectedGraph\DirectedGraph.csproj.FileListAbsolute.txt". The process cannot access the file 'D:\j\workspace\x64_release_w---0575cb46\bin\int\Managed\GC\Stress\Tests\DirectedGraph\DirectedGraph.csproj.FileListAbsolute.txt' because it is being used by another process. [D:\j\workspace\x64_release_w---0575cb46\tests\src\GC\Stress\Tests\DirectedGraph.csproj]
07:39:33 Command execution failed with exit code 1.
07:39:33 BUILDTEST: Error: build failed. Refer to the build log files for details:

Is this a known failure? Is it common? Is it being fixed?

https://ci.dot.net/job/dotnet_coreclr/job/master/job/x64_release_windows_nt_pri1_prtest/549/

[BruceForstall]

@dotnet-bot test Windows_NT x64 Release Priority 1 Build and Test

[swgillespie]

@BruceForstall It's known, yes: https://github.com/dotnet/coreclr/issues/7762

[pgavlin]

LGTM