[mini] Fix typo in mono_decompose_vtype_opts

[lambdageek]

Without this, if some previous instruction already created a vreg for ins->dest (for example if we are doing multiple passes over the basic block because restart == TRUE) we will use an incorrect vreg for the src when decomposing the current VMOVE

---

Additionally, only emit OP_LDTOKEN_FIELD if we loaded a field token

This is used by a CreateSpan intrinsic #81695 that needs access to the MonoClassField*

For other cases of a bare LDTOKEN (such as hand-written IL that calls LDTOKEN on a type but doesn't follow it up with a call to GetTypeFromHandle leave the opcode as a VMOVE (from the EMIT_NEW_TEMPLOAD above))

Fixes #90800

[lambdageek]

TODO: need to add a regression test.

attn @SamMonoRT we will need to backport this to net8.

[lambdageek]

@simonrozsival @rolfbjarne do we need to try to get this into rc1, is it completely blocking ios?

As a workaround, emitting roslyn-style IL will avoid this issue (because there will be an intervening callvirt that will avoid the decompose bug)
https://sharplab.io/#v2:C4LglgNgPgAgTARgLACgDEA7ArhCBDAIwgFMACAEzAGdCTVSHTUYBmU+UgYVIG97HWpAEpYMwMAFtiAFQCeAB2IAJPBnIlSAWQAUASl79GAgOylgC4gHsAZtpgIADLoB0cxSrUkA3IdIBfVD8gA=

ie instead of ldtoken System.Runtime]System.String emit

        IL_0000: ldtoken [System.Runtime]System.String
        IL_0005: call class [System.Runtime]System.Type [System.Runtime]System.Type::GetTypeFromHandle(valuetype [System.Runtime]System.RuntimeTypeHandle)
        IL_000a: callvirt instance valuetype [System.Runtime]System.RuntimeTypeHandle [System.Runtime]System.Type::get_TypeHandle()

[lewing]

if this is a blocker for rc1 we should open the backport now and send the email

[lambdageek]

/backport to release/8.0-rc1

[github-actions]

Started backporting to release/8.0-rc1: https://github.com/dotnet/runtime/actions/runs/5907349234

[lambdageek]

@lewing I'm offline for at least another 3 hours. Can you send the email - see the comments in the issue for details I sent the email. Waiting on a decision...