mips32/mips64: Fix the problem with lwl and lwr for mips32 and mips64.

This patch is fixing the problem with lwl and lwr for mips32 and mips64
that was discovered while running v8 tests for mips32.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2749 8f6e269a-dfd6-0310-a8e1-e2731360e62c