fix int-to-long conversions from memory on x86_32

The instruction for 32-bit-to-64-bit sign extension on x86_32 requires
that the input value be placed in EAX and the sign extension in EDX.
However, the compiler can get confused if the input value is in memory
addressed via one of those registers and doesn't know how to move it.
This patch works around that limitation by doing the move explicitly
in MemoryEvent::compile if necessary.