Fix Arm64 build breaks

Fix Arm64 build breaks

The `GT_STORE_OBJ` case in codegen was missing a break.
`getStructHandleIfPresent` wasn't correctly handling `GT_ASG'.
The `srcCount` was not being set correctly for block ops with a constant size.
And I lost some code in `fgMakeTmpArgNode` with a merge.
Also, I had introduced a dumping issue in gentree.cpp when I made `GT_OBJ` a `GTK_UNOP` as a result of some PR comments.

Commit migrated from https://github.com/dotnet/coreclr/commit/fedf90c774d63c0423858f2948cb63d2dcd9de3a