if vpush/vpop needs to emit multiple instructions (because of
non-consecutive registers), then the emitted sequence should be:

vpush list1
vpush list2
...
vpop list2
vpop list1

Subzero was emiting vpop in the wrong order:

vpop list1
vpop list2

These multiple lists push/pop arise because of the way fp32 and fp64
registers are declared (s0 -> s31, d31 -> d0).

This CL modifies fp64 registers so they are declared in ascending
order (d0 -> d31), which fixes subzero temporarily. The appropriate
fix is to change vpop to be emitted in the right order.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4076
R=sehr@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1592663004 .