Improve codegen for initializer lists to use memset more aggressively

Improve codegen for initializer lists to use memset more aggressively
when an initializer is variable (I handled the constant case in a previous
patch).  This has three pieces:

1. Enhance AggValueSlot to have a 'isZeroed' bit to tell CGExprAgg that
   the memory being stored into has previously been memset to zero.
2. Teach CGExprAgg to not emit stores of zero to isZeroed memory.
3. Teach CodeGenFunction::EmitAggExpr to scan initializers to determine
   whether they are profitable to emit a memset + inividual stores vs
   stores for everything.

The heuristic used is that a global has to be more than 16 bytes and
has to be 3/4 zero to be candidate for this xform.  The two testcases
are illustrative of the scenarios this catches.  We now codegen test9 into:

 call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 400, i32 4, i1 false)
 %.array = getelementptr inbounds [100 x i32]* %Arr, i32 0, i32 0
 %tmp = load i32* %X.addr, align 4
 store i32 %tmp, i32* %.array

and test10 into:

  call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 392, i32 8, i1 false)
  %tmp = getelementptr inbounds %struct.b* %S, i32 0, i32 0
  %tmp1 = getelementptr inbounds %struct.a* %tmp, i32 0, i32 0
  %tmp2 = load i32* %X.addr, align 4
  store i32 %tmp2, i32* %tmp1, align 4
  %tmp5 = getelementptr inbounds %struct.b* %S, i32 0, i32 3
  %tmp10 = getelementptr inbounds %struct.a* %tmp5, i32 0, i32 4
  %tmp11 = load i32* %X.addr, align 4
  store i32 %tmp11, i32* %tmp10, align 4

Previously we produced 99 stores of zero for test9 and also tons for test10.
This xforms should substantially speed up -O0 builds when it kicks in as well
as reducing code size and optimizer heartburn on insane cases.  This resolves
PR279.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120692 91177308-0d34-0410-b5e6-96231b3b80d8