Partition allocas that occur in the entry block into two categories.  The first is those whose size is fixed and alignment are less than or equal to the stack alignment.  These are emitted relative to a pointer, either in increasing offset relative to the stack pointer or decreasing offset relative to the frame pointer.  (Actually, we are not enabling this optimization for frame pointer frames yet)  The second category is allocas whose size is dynamic or alignment is creater than the stack alignment.  These are emitted relative to a user variable in increasing offset order.  This optimization is only enabled for x86 at O2.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1411583007 .