x64: Fix LMathMinMax for constant Smi right-hand operands

BUG=chromium:349079
LOG=y
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/186593003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19668 ce2b1a6d-e550-0410-aec6-3dcde31c8c00