Fix morphing of block op assignments

Fix morphing of block op assignments

During morphing, block op assignments had their children set to
the "IND" morph context. This is incorrect. Previously, block
ops would "incorporate" the "IND-ness" of their children. However,
now, the operands will be a IND/BLK/OBJ node explicitly. Deleting
this incorrect code enables other changes, not yet merged, to work.

This causes no amd64 SPMI desktop asm diffs.

Commit migrated from https://github.com/dotnet/coreclr/commit/87ebd5f4fca70a50d51e5503d13b4e5a05d8d712