[turbofan] Add TruncateFloat64ToInt32 machine operator.

Fix ChangeLowering to use TruncateFloat64ToInt32.

TEST=cctest,compiler-unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/484103002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23214 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
[turbofan] Add support for ChangeUint32ToTagged in ChangeLowering.

TEST=compiler-unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/491433002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23215 ce2b1a6d-e550-0410-aec6-3dcde31c8c00