Consolidate importer spilling code V2 (#72744)

Consolidate importer spilling code V2 (#72744)

* 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'

* Fix assignments done via return buffers

The mistake in logic was that the only trees which could modify
unaliased locals are assignments, which is not true, calls can
do that as well.

One day we will move the return buffer handling out of importer,
but until then, special handling is required.

An alternative fix would have been to bring back the explicit
"impSpillLclRefs" to "stloc/starg" code, but that would contradict
the overall goal of consolidating the spilling logic.