Fix map check removal issue in r13488.

The removal of HCheckMaps is invalid when the instruction has a virtual
usage, which can happen e.g. for HLoadElements or HJSArrayLength.

R=jkummerow@chromium.org
TEST=webkit:fast/js/regress/Float32Array-matrix-mult.html

Review URL: https://codereview.chromium.org/12033073

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13491 ce2b1a6d-e550-0410-aec6-3dcde31c8c00