Improve insn selection for helper calls. Attempt to evaluate arguments

Improve insn selection for helper calls. Attempt to evaluate arguments
into the real register that is mandated by the ABI instead of evaluating
it in a virtual register and then move the result.
Observed savings in insns between 0.5% and 1.4%.
Probably an overrated optimization given current helper functions which
rarely take more than one argument.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2326