Unify alloca, outgoing arg, and prolog construction

tests_lit/assembler/x86/sandboxing.ll

 ; Tests basics and corner cases of x86-32 sandboxing, using -Om1 in
 ; the hope that the output will remain stable.  When packing bundles,
 ; we try to limit to a few instructions with well known sizes and
 ; minimal use of registers and stack slots in the lowering sequence.
 
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -Om1 \
 ; RUN:   -allow-externally-defined-symbols \
 ; RUN:   -ffunction-sections -sandbox | FileCheck %s
 
 declare void @call_target()
@@ skipping 233 matching lines @@
 ; observed by the stack adjustment for accessing stack-allocated
 ; variables.
 define internal void @checkpoint_restore_stack_adjustment(i32 %arg) {
 entry:
   call void @call_target()
   ; bundle boundary
   call void @checkpoint_restore_stack_adjustment(i32 %arg)
   ret void
 }
 ; CHECK-LABEL: checkpoint_restore_stack_adjustment
+; CHECK: sub esp,0x1c
 ; CHECK: call
-; CHECK: sub esp,0x10
 ; The address of %arg should be [esp+0x20], not [esp+0x30].
 ; CHECK-NEXT: mov [[REG:.*]],DWORD PTR [esp+0x20]
 ; CHECK-NEXT: mov DWORD PTR [esp],[[REG]]
 ; CHECK: call
-; CHECK: add esp,0x10
+; CHECK: add esp,0x1c