ARM: Tweak ECMAToInt32VFP to address regression on Nexus 4

After r13912, we saw a small regression in Kraken crypto-aes and
crypto-ccm on Nexus 4. This patch, proposed by Rodolph Perfetta,
addresses that without regressing other platforms.

Instead of looking at the exponent of double_input and trying to figure
out if the conversion will overflow, eagerly do the VCVT and return
early unless it saturated.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/12544025
Patch from Hans Wennborg <hans@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13948 ce2b1a6d-e550-0410-aec6-3dcde31c8c00