If alignment parameter is set, the memory returned by the
StackSlot operator will be aligned according to the parameter.

The implementation goes like this. If alignment parameter is set
we allocate a bit more memory than actually needed and so we
can move the beginning of the StackSlot in order to have it aligned.

BUG=

Review-Url: https://codereview.chromium.org/2816743003
Cr-Commit-Position: refs/heads/master@{#45197}