Adds support for variable sized allocas. For a variable sized alloca,

code is inserted to first check if the current stacklet has enough
space. If so, space is allocated by simply decrementing the stack
pointer. Otherwise a runtime routine (__morestack_allocate_stack_space
in libgcc) is called which allocates the required memory from the
heap.

Patch by Sanjoy Das.

llvm-svn: 138818