Teach the x86 back end how generate 'lea' instructions, and generate

Teach the x86 back end how generate 'lea' instructions, and generate
them in a couple of places which are important.  This reduces the
amount of generated code for memcheck and none by about 1%, and (in
very unscientific tests on perf/bz2) speeds memcheck up by about 1%.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1745