mips: Clean up type overrides for operands.

mips: Clean up type overrides for operands.

For operands which default to uw (uint32_t), there's no reason to
explicitly specify that all over the place. Also, when assigning to a
32 bit value which is supposed to be the full width of the resulting
register, there's no reason to override the value to be signed. If the
value is expanded into a larger value, then extra bits may get set
unintentionally through sign extension. Even if an instruction
produces a value which should be interpreted as signed, it will still
only produce a value of a certain predefined width, even if that answer
ends up stored in a larger variable.

Change-Id: I048d68c5dd08a1d40e8117ae9d36d70e05ec21c8
Reviewed-on: https://gem5-review.googlesource.com/c/13618
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>