In block morphing, "addrSpill" is used when the destination or source
represent indirections of "complex" addresses. Unfortunately, some trees
in the form of "IND(ADDR(LCL))" fall into this category.

If such an "ADDR(LCL)" is used as an "addrSpill", the underlying local
*must* be marked as address-exposed. Block morphing was using a very
simplistic test for when that needs to happen, essentially only recognizing
"ADDR(LCL_VAR/FLD)". But it is possible to have a more complicated pattern
as "PrepareDst/Src" uses "IsLocalAddrExpr" to recognize indirect stores
to locals.

Currently it appears impossible to get a mismatch here as morph transforms
"IND(ADD(ADDR(LCL_VAR), OFFSET))" into "LCL_FLD" (including for TYP_STRUCT
indirections), but this is a very fragile invariant. Transforming TYP_STRUCT
GT_FIELDs into GT_OBJs instead of GT_INDs breaks it, for example.

Fix this by address-exposing the local obtained via "IsLocalAddrExpr".