Improvements to code generation for 32 bit instructions. When

Improvements to code generation for 32 bit instructions.  When
appropriate, generate 32 bit add/sub/and/or/xor/cmp, so as to avoid a
bunch of cases where previously values would have been widened to 64
bits, or shifted left 32 bits, before being used.  Reduces the size of
the generated code by up to 2.8%.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2156