Fix for a smi stores optimization on x64 with a regression test.

BUG=345715
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/178833002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19535 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Don't eliminate loads with incompatible types or representations.

BUG=346343
LOG=y
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/179553002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19536 ce2b1a6d-e550-0410-aec6-3dcde31c8c00