[Stack realignment] Handling of aligned allocas.

[Stack realignment] Handling of aligned allocas.

This patch implements dynamic realignment of stack objects for targets
with a non-realigned stack pointer. Behaviour in FunctionLoweringInfo
is changed so that for a target that has StackRealignable set to
false, over-aligned static allocas are considered to be variable-sized
objects and are handled with DYNAMIC_STACKALLOC nodes.

It would be good to group aligned allocas into a single big alloca as
an optimization, but this is yet todo.

SystemZ benefits from this, due to its stack frame layout.

New tests SystemZ/alloca-03.ll for aligned allocas, and
SystemZ/alloca-04.ll for "no-realign-stack" attribute on functions.

Review and help from Ulrich Weigand and Hal Finkel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254227 91177308-0d34-0410-b5e6-96231b3b80d8