MIPS: Fix debug mode webkit failures.

* Handles SMI case in EmitLoadRegister correctly.
* Fix minor mistake in the assembler.

BUG=v8:2808
TEST=webkit/dfg-dead-min-two-args,webkit/fast/js/excessive-comma-usage

Review URL: https://codereview.chromium.org/20801003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15914 ce2b1a6d-e550-0410-aec6-3dcde31c8c00