Valgrind used to report errors about uninitialized variable access in
Subzero, when it was built with -O2. The problem was traced to

size_t Alignment = Var->getAlignment;
Alignment = std::max(MinAlignment, Var->getAlignment())

Apparently, the compiler will not correctly zero-extend
Var->getAlignment(), and thus Alignment's upper 32-bits would be
garbage.

BUG=
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1825363003 .