Fix some problems with HHIR generic memory ops that take C(Int) sources
Summary: The pointee() function was ignoring possible offsets, so just refactor
things so we always use offset zero with these. Also add a DbgTrashMem. (I
noticed this problem partially through adding that.) The packedarray functions
were handling it properly. LdElem and StElem are used with non-constant
integer offsets---I want to replace them with a new typed-lea and StMem/LdMem,
but for this diff just make them not incorrect they way they are.
Reviewed By: @alexmalyshev
Differential Revision:
D1772985