After some time of being neglected, this CL improves FP lowering for
ARM32.

1) It emits vpush {list}, and vpop {list} when possible.

2) It stops saving alised Vfp registers multiple times (yes, sz used to
save both D and S registers even when they aliased.)

3) Introduces Vmla (fp multiply and accumulate) and Vmls (multiply and
subtract.)

(1 + 2) minimally (but positively) affected SPEC.

(3) caused a 2% geomean improvement.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1481133002 .