[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. llvm-svn: 192119
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