[GVNHoist] Merge DebugLoc metadata on hoisted instructions

Summary:
When instructions are hoisted, current implementation keeps DebugLoc metadata of the instruction that chosen as Repl (and its GEP operand if Repl is a load or a store). However, DebugLoc metadata should be updated to the 'merged' location across all hoisted instructions. See the following example code:


```
  1:  typedef struct {
  2:    int a[10];
  3:  } S1;
  4: 
  5:  extern S1 *s1[10];
  6: 
  7:  void foo(int x, int y, int i) {
  8:    if (y)
  9:      s1[i]->a[i] = x + y;
 10:    else
 11:      s1[i]->a[i] = x;
 12:  }
```

Below is LLVM IR representation of the program before gvn-hoist:


```
%struct.S1 = type { [10 x i32] }
@s1 = external local_unnamed_addr global [10 x %struct.S1*], align 16

define void @foo(i32 %x, i32 %y, i32 %i) !dbg !4 {
entry:
  %tobool = icmp ne i32 %y, 0, !dbg !8
  br i1 %tobool, label %if.then, label %if.else, !dbg !10

if.then:                                          ; preds = %entry
  %add = add nsw i32 %x, %y, !dbg !11
  %idxprom = sext i32 %i to i64, !dbg !12
  %arrayidx = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !12
  %0 = load %struct.S1*, %struct.S1** %arrayidx, align 8, !dbg !12, !tbaa !13
  %a = getelementptr inbounds %struct.S1, %struct.S1* %0, i32 0, i32 0, !dbg !17
  br label %if.end, !dbg !12

if.else:                                          ; preds = %entry
  %idxprom3 = sext i32 %i to i64, !dbg !18
  %arrayidx4 = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom3, !dbg !18
  %1 = load %struct.S1*, %struct.S1** %arrayidx4, align 8, !dbg !18, !tbaa !13
  %a5 = getelementptr inbounds %struct.S1, %struct.S1* %1, i32 0, i32 0, !dbg !19
  br label %if.end

if.end:                                           ; preds = %if.else, %if.then
  %a5.sink = phi [10 x i32]* [ %a5, %if.else ], [ %a, %if.then ]
  %.sink = phi i32 [ %x, %if.else ], [ %add, %if.then ]
  %idxprom6 = sext i32 %i to i64
  %arrayidx7 = getelementptr inbounds [10 x i32], [10 x i32]* %a5.sink, i64 0, i64 %idxprom6
  store i32 %.sink, i32* %arrayidx7, align 4, !tbaa !20
  ret void, !dbg !22
}

```
where


```
!11 = !DILocation(line: 9, column: 18, scope: !9)
!12 = !DILocation(line: 9, column: 5, scope: !9)
!18 = !DILocation(line: 11, column: 5, scope: !9)
!19 = !DILocation(line: 11, column: 9, scope: !9)
```

. And below is after gvn-hoist:


```
define void @foo(i32 %x, i32 %y, i32 %i) !dbg !4 {
entry:
  %tobool = icmp ne i32 %y, 0, !dbg !8
  %idxprom = sext i32 %i to i64, !dbg !10
  %0 = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !10
  %1 = load %struct.S1*, %struct.S1** %0, align 8, !dbg !10, !tbaa !11
  br i1 %tobool, label %if.then, label %if.else, !dbg !15

if.then:                                          ; preds = %entry
  %add = add nsw i32 %x, %y, !dbg !16
  %arrayidx = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !10
  %a = getelementptr inbounds %struct.S1, %struct.S1* %1, i32 0, i32 0, !dbg !17
  br label %if.end, !dbg !10

if.else:                                          ; preds = %entry
  %arrayidx4 = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !18
  %a5 = getelementptr inbounds %struct.S1, %struct.S1* %1, i32 0, i32 0, !dbg !19
  br label %if.end

if.end:                                           ; preds = %if.else, %if.then
  %a5.sink = phi [10 x i32]* [ %a5, %if.else ], [ %a, %if.then ]
  %.sink = phi i32 [ %x, %if.else ], [ %add, %if.then ]
  %arrayidx7 = getelementptr inbounds [10 x i32], [10 x i32]* %a5.sink, i64 0, i64 %idxprom
  store i32 %.sink, i32* %arrayidx7, align 4, !tbaa !20
  ret void, !dbg !22
}

```
As you see, loads and their GEPs have been hosited from if.then/if.else block to entry block. However, DebugLoc metadata of these new instructions are still same as the instructions in if.then block, as they are moved/cloned from if.then block. This may result incorrect stepping and imprecise sample profile result.

Reviewers: majnemer, pcc, sebpop

Reviewed By: sebpop

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29377

llvm-svn: 294250