MIPS64: Add DMULT and DMULTU instr to simulator.

This is a quick hack that needs fixing.

Also, there are mips64 rules about how the 32-bit MULT/MULTU
work with 32-bit inputs, which should be validated with ASSERTS.
MIPS64: Fix DoLoadNamedField() when the field is an Smi.

If the field is Smi but our representation is integer32,
then load the value directly from the upper 32bits of
the 64-bit Smi.
MIPS64: Fix some representation-related issues.
MIPS64: Disassemble the stop(msg) psuedo-instruction correctly.

The stop(msg) embeds a raw 64-bit pointer in the code. But the
disassembler would choke & die on those raw bytes, crashing 
with UNIMPLEMENTED or UNREACHABLE.

This prevented using --print-all-code on many tests.