When translating an Variable without a register, the code assumes the
variable is on the stack using sp. This is still true. However, if it
is a (derived class) StackVariable, the register defined by
getBaseRegNum() should be used instead.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1432453003 .