[PATCH] [SROA] Fold a PHI node if all its incoming values are the same

[Jingyue Wu]

Hi Duncan,

I tried SimplifyPHINode and it worked pretty well. Thanks! 

That makes me consider using SimplifySelectInst on select instructions too. However, I found one regression test PR16651.2 would fail after this potential modification. We would transform

```
define void @PR16651.2() {
; This test case caused a crash due to failing to promote given a select that
; can't be speculated. It shouldn't be promoted, but we missed that fact when
; analyzing whether we could form a vector promotion because that code didn't
; bail on select instructions.
;
; CHECK-LABEL: @PR16651.2(
; CHECK: alloca <2 x float>
; CHECK: ret void

entry:
  %tv1 = alloca { <2 x float>, <2 x float> }, align 8
  %0 = getelementptr { <2 x float>, <2 x float> }* %tv1, i64 0, i32 1
  store <2 x float> undef, <2 x float>* %0, align 8
  %1 = getelementptr inbounds { <2 x float>, <2 x float> }* %tv1, i64 0, i32 1, i64 0
  %cond105.in.i.i = select i1 undef, float* null, float* %1
  %cond105.i.i = load float* %cond105.in.i.i, align 8
  ret void
}
```
to
```
define void @PR16651.2() {
entry:
  %cond105.in.i.i = select i1 undef, float* null, float* undef
  %cond105.i.i = load float* %cond105.in.i.i, align 8
  ret void
}
```

Is this transformation on PR16651.2 valid? If no, can somebody help me understand why it isn't? 

Thanks,
Jingyue

http://reviews.llvm.org/D4659