[mips] Fix definition of mfhi and mflo instructions to read from the whole
accumulator instead of its sub-registers, $hi and $lo.

We need this change to prevent a mflo following a mtlo from reading an
unpredictable/undefined value, as shown in the following example:

mult $6, $7 // result of $6 * $7 is written to $lo and $hi.
mflo $2     // read lower 32-bit result from $lo.
mtlo $4     // write to $lo. the content of $hi becomes unpredictable.
mfhi $3     // read higher 32-bit from $hi, which has an unpredictable value.

I don't have a test case for this change that reliably reproduces the problem.