Since push isn't used for args passing anymore, the cases of handling
push for vectors and floats/doubles isn't needed anymore. Passing
vectors requires a bit more care of alignment, so that was changed.

I can imagine push needing to handle addresses later (at least on
x86-64 to push the lower 32-bits of return address), but for now,
this means only handling GPRs. The XMM registers are not callee saved.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/633553003