[release/8.0-rc1] [mini] Fix typo in mono_decompose_vtype_opts (#90832)

* Fix typo in mono_decompose_vtype_opts

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 when
decomposing the current VMOVE

Fixes #90800

* Only emit an OP_LDTOKEN_FIELD if we loaded a field token

This is used by a CreateSpan optimization 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))

---------

Co-authored-by: Aleksey Kliger <alklig@microsoft.com>

src/mono/mono/mini/decompose.c

@@ -1226,7 +1226,7 @@ mono_decompose_vtype_opts (MonoCompile *cfg)
 					dest_var = get_vreg_to_inst (cfg, ins->dreg);
 
 					if (!src_var)
-						src_var = mono_compile_create_var_for_vreg (cfg, m_class_get_byval_arg (ins->klass), OP_LOCAL, ins->dreg);
+						src_var = mono_compile_create_var_for_vreg (cfg, m_class_get_byval_arg (ins->klass), OP_LOCAL, ins->sreg1);
 
 					if (!dest_var)
 						dest_var = mono_compile_create_var_for_vreg (cfg, m_class_get_byval_arg (ins->klass), OP_LOCAL, ins->dreg);

src/mono/mono/mini/method-to-ir.c

@@ -10840,12 +10840,15 @@ mono_method_to_ir (MonoCompile *cfg, MonoMethod *method, MonoBasicBlock *start_b
 					EMIT_NEW_TEMPLOADA (cfg, addr, vtvar->inst_c0);
 					MONO_EMIT_NEW_STORE_MEMBASE (cfg, OP_STORE_MEMBASE_REG, addr->dreg, 0, ins->dreg);
 					EMIT_NEW_TEMPLOAD (cfg, ins, vtvar->inst_c0);
-					ins->opcode = OP_LDTOKEN_FIELD;
-					ins->inst_c0 = n;
-					ins->inst_p1 = handle;
+					if (handle_class == mono_defaults.fieldhandle_class) {
+						ins->opcode = OP_LDTOKEN_FIELD;
+						ins->inst_c0 = n;
+						ins->inst_p1 = handle;
+
+						cfg->flags |= MONO_CFG_NEEDS_DECOMPOSE;
+						cfg->cbb->needs_decompose = TRUE;
+					}
 
-					cfg->flags |= MONO_CFG_NEEDS_DECOMPOSE;
-					cfg->cbb->needs_decompose = TRUE;
 				}
 			}