Backport r192119 to fix Clang/MIPS reads from whole accumulator instead of HI/LO

  commit 243702b9
  Author: Akira Hatanaka <ahatanaka@mips.com>
  Date:   Mon Oct 7 18:49:46 2013 +0000

    [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.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20131007/190249.html

Change-Id: I1052bb9b8fa575d29df8e27d9cd837adc57d1189