[MonoVM] Fix assert on startup with abcrem optimization enabled

[filipnavara]

Match the logic in mini_emit_sext_index_reg and OP_BOUNDS_CHECK decomposition. This was already done for LLVM but not for any other target. Instead there was an assert which was hit early on startup when the optimization was enabled (tested on ARM64).

The optimization is not enabled by default but that's separate issue. There are potentially some performance wins to have. For example, the Burgers benchmarks in dotnet/performance shows this:

Default:

| Method    | Mean       | Error   | StdDev  | Median     | Min        | Max        | Gen0       | Gen1       | Gen2       | Allocated    |
|---------- |-----------:|--------:|--------:|-----------:|-----------:|-----------:|-----------:|-----------:|-----------:|-------------:|
| Burgers_3 |   487.9 ms | 2.33 ms | 2.06 ms |   488.2 ms |   483.5 ms |   490.7 ms |          - |          - |          - |    156.76 KB |
| Burgers_0 | 1,266.8 ms | 3.19 ms | 2.83 ms | 1,265.5 ms | 1,264.5 ms | 1,273.6 ms | 47000.0000 | 47000.0000 | 47000.0000 | 781750.05 KB |
| Burgers_1 | 1,180.1 ms | 2.98 ms | 2.49 ms | 1,180.4 ms | 1,176.5 ms | 1,185.3 ms |          - |          - |          - |    156.74 KB |
| Burgers_2 |   286.9 ms | 0.17 ms | 0.15 ms |   286.9 ms |   286.6 ms |   287.0 ms |          - |          - |          - |    156.74 KB |

With -O=abcrem:

| Method    | Mean       | Error   | StdDev  | Median     | Min        | Max        | Gen0       | Gen1       | Gen2       | Allocated    |
|---------- |-----------:|--------:|--------:|-----------:|-----------:|-----------:|-----------:|-----------:|-----------:|-------------:|
| Burgers_3 |   395.5 ms | 1.25 ms | 1.11 ms |   395.6 ms |   394.0 ms |   397.8 ms |          - |          - |          - |    156.76 KB |
| Burgers_0 | 1,167.8 ms | 5.45 ms | 4.83 ms | 1,167.5 ms | 1,161.5 ms | 1,176.8 ms | 47000.0000 | 47000.0000 | 47000.0000 | 781750.05 KB |
| Burgers_1 | 1,104.2 ms | 2.82 ms | 2.64 ms | 1,102.9 ms | 1,100.4 ms | 1,108.5 ms |          - |          - |          - |    156.74 KB |
| Burgers_2 |   192.2 ms | 0.36 ms | 0.30 ms |   192.2 ms |   191.8 ms |   192.8 ms |          - |          - |          - |    156.74 KB |

[steveisok]

I'm ok w/ it assuming the assert is no longer necessary for what we ship.

[filipnavara]

It just moves the need_sext handling one level up from the LLVM code branch so it applies to all targets, so we don't need the assert anymore. It's currently not used in any shipping code AFAIK.