emitIAS for push -- simplify push since it's not used for args passing anymore

emitIAS for push -- simplify push since it's not used for args passing anymore

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

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=0b9eee5221afe4c074e51313cb272556bb1415bc

[jvoung (off chromium), 2014-10-06 14:56:51]

jvoung@chromium.org changed reviewers:
+ stichnot@chromium.org

[Jim Stichnoth, 2014-10-06 15:09:21]

LGTM.

I envision at least one future use of push/pop:
1. Do normal lowering and register allocation before Phi lowering.
2. Do Phi lowering by splitting critical edges and putting assignments in the
new node.
3. Do lowering / register allocation in those new nodes.

In the last step, it's possible that there are no free registers for lowering
the assignments, so we would have to create one by spilling one to the stack.

My first thought would be to do this via push/pop, but I guess we could also do
this by creating an explicit spill variable and using mov instructions.  These
push/pops would only involve physical registers, but that could include xmm/FP
registers.

[jvoung (off chromium), 2014-10-07 18:20:15]

Committed patchset #1 (id:1) manually as
0b9eee5221afe4c074e51313cb272556bb1415bc (presubmit successful).