[CaptureTracking] Do not check domination

For the CapturesBefore tracker, it is sufficient to check that
I can not reach BeforeHere. This does not necessarily require
that BeforeHere dominates I, it can also occur if the capture
happens on an entirely disjoint path.

This change was previously accepted in D90688, but had to be
reverted due to large compile-time impact in some cases: It
increases the number of reachability queries that are performed.

After recent changes, the compile-time impact is largely mitigated,
so I'm reapplying this patch. The remaining compile-time impact
is largely proportional to changes in code-size.

llvm/lib/Analysis/CaptureTracking.cpp

@@ -143,14 +143,8 @@ namespace {
         return !isPotentiallyReachableFromMany(Worklist, BB, nullptr, DT);
       }
 
-      // If the value is defined in the same basic block as use and BeforeHere,
-      // there is no need to explore the use if BeforeHere dominates use.
       // Check whether there is a path from I to BeforeHere.
-      if (DT->dominates(BeforeHere, I) &&
-          !isPotentiallyReachable(I, BeforeHere, nullptr, DT))
-        return true;
-
-      return false;
+      return !isPotentiallyReachable(I, BeforeHere, nullptr, DT);
     }
 
     bool captured(const Use *U) override {

llvm/test/Transforms/MemCpyOpt/callslot.ll

@@ -252,8 +252,8 @@ define void @capture_nopath_call(i1 %cond) {
 ; CHECK-NEXT:    call void @accept_ptr(i8* [[DEST_I8]])
 ; CHECK-NEXT:    ret void
 ; CHECK:       nocaptures:
-; CHECK-NEXT:    call void @accept_ptr(i8* [[SRC_I8]]) #[[ATTR3]]
-; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DEST_I8]], i8* [[SRC_I8]], i64 16, i1 false)
+; CHECK-NEXT:    [[DEST1:%.*]] = bitcast [16 x i8]* [[DEST]] to i8*
+; CHECK-NEXT:    call void @accept_ptr(i8* [[DEST1]]) #[[ATTR3]]
 ; CHECK-NEXT:    ret void
 ;
   %dest = alloca [16 x i8]