[PEI, AArch64] Use empty spaces in stack area for local stack slot allocation.

Summary:
If the target requests it, use emptry spaces in the fixed and
callee-save stack area to allocate local stack objects.

AArch64: Change last callee-save reg stack object alignment instead of
size to leave a gap to take advantage of above change.

Reviewers: t.p.northover, qcolombet, MatzeB

Subscribers: rengolin, mcrosier, llvm-commits, aemerson

Differential Revision: http://reviews.llvm.org/D20220

llvm-svn: 271527
diff --git a/llvm/test/CodeGen/AArch64/aarch64-dynamic-stack-layout.ll b/llvm/test/CodeGen/AArch64/aarch64-dynamic-stack-layout.ll
index 9429e87..708ae08 100644
--- a/llvm/test/CodeGen/AArch64/aarch64-dynamic-stack-layout.ll
+++ b/llvm/test/CodeGen/AArch64/aarch64-dynamic-stack-layout.ll
@@ -674,7 +674,7 @@
 
 define void @realign_conditional2(i1 %b) {
 entry:
-  %tmp = alloca i8, i32 4
+  %tmp = alloca i8, i32 16
   br i1 %b, label %bb0, label %bb1
 
 bb0: