[Hexagon] Generate store-immediate instructions for stack objects

[Hexagon] Generate store-immediate instructions for stack objects

Store-immediate instructions have a non-extendable offset. Since the
actual offset for a stack object is not known until much later, only
generate these stores when the stack size (at the time of instruction
selection) is small.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305305 91177308-0d34-0410-b5e6-96231b3b80d8