MIPS: Fix test-simplified-lowering to pass without tf implemention.

GenerateCode() only for SupportedTarget() in 3 places, as in the
other tests here.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/459913002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23055 ce2b1a6d-e550-0410-aec6-3dcde31c8c00