Split and replace the EmitVFPTruncate routine to only do what is needed. Floor
and Round have been rewritten to use the default rounding mode.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/12393008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13833 ce2b1a6d-e550-0410-aec6-3dcde31c8c00