Consolidate importer spilling code (#72291)

Consolidate importer spilling code (#72291)

* Add tests

* Fix losing GLOB_REF on the LHS

The comment states we don't need it, which is incorrect.

Diffs are improvements because we block forward substitution of
calls into "ASG(BLK(ADDR(LCL_VAR<field>, ...)))", which allows
morph to leave the "can be replaced with its field" local alone.

* Prospective fix

Spill "glob refs" on stores to "aliased" locals.

* Delete now-not-necessary code

* Fix up asserts

* Clean out '(unsigned)CHECK_SPILL_ALL/NONE' casts

* Don't manually spill for 'st[s]fld'

* Revert 'Clean out '(unsigned)CHECK_SPILL_ALL/NONE' casts'