Handle lowering of "trunc i64 to ..." instruction.  Refs #13.
Handle lowering of "sext/zext i64 to ..." instructions.  Refs #13.
Fix lowerAssign() to handle I64 assigns.  Refs #13.

Extends IceConstantRelocatable to include a translation-time constant
offset.  Updates the CHECK: output in convert.ll to be up to date,
though there will be more changes after I64 Store lowering is
implemented.