As the register allocator cannot reuse spill slots, SameAsFirst
constraint means that we would have to do an expensive move to a
differen spill slot if we choose to spill. Forcing the operand
to a register is cheaper.

In zlib, we get >10% speed-up for ia32, >25% for x64.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/650083003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24580 ce2b1a6d-e550-0410-aec6-3dcde31c8c00