Stop hoisting statics above cctors

Stop hoisting statics above cctors

Extend the logic in optLoopHoist around `CLS_VAR` nodes to ensure we don't
hoist any tree that contains a `CLS_VAR` for a field with the flag
`CORINFO_FLG_FIELD_INITCLASS` set, unless we also (identify and) hoist the
corresponding static init helper call.  The previous logic was
insufficient in that it blocked hoisting of singleton `CLS_VAR` nodes, but
not hoisting of trees that contain `CLS_VAR` nodes as sub-trees.

Add flag `GTF_FLD_INITCLASS`/`GTF_CLS_VAR_INITCLASS` so that optLoopHoist
can recall which fields are paired with static init helper calls, and only
block the hoisting of their trees.

Fixes #10780.