[Mem2Reg] Skip load [take] of cast projections.
Already, load [take]s of struct_element_addr|tuple_element_addr
projections resulted in Mem2Reg bailing. Expand that to include load
[take]s involving unchecked_addr_cast.
To handle load [take]s of (struct|tuple)_element_addr projections, it
would be necessary to replace the running value with a value obtained
from the original product by recursive destructuring, replacing the
value at the load [take]n address with undef, and then restructuring.
To handle load [take]s of cast projections, it would be necessary to use
unchecked_value_cast instead of unchecked_bitwise_cast. But we would
need to still use unchecked_bitwise_cast in the case of load [copy]
because otherwise we would lose the original value--unchecked_value_cast
forwards ownership, and not all casts can be reversed (because they may
narrow).
For now, just bail out in the face of these complex load [take]s.
