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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138818 91177308-0d34-0410-b5e6-96231b3b80d8