Change the i1 zeroext parameter to an explicit zext and
i32. Add an assert in lowerCall that the type is at least
32-bits.

I ended up putting the assert in lowerCall instead of
InstX8632Push, since technically there are quite a few
modes that push allows: 16-bit reg/mem (just not 8-bit
reg/mem) and 8/16/32 bit constants.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/339933004