[AMDGPU] Fix excessive stack usage in SIInsertWaitcnts::run (#134835)

Noticed on Windows when running LLVM as part of a graphics driver, with
total stack usage limited to about 128 KB. In some cases this function
would overflow the stack.

On Linux this reduces stack usage in this function from about 32 KB to
about 0.5 KB.

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

@@ -2623,12 +2623,17 @@ bool SIInsertWaitcnts::run(MachineFunction &MF) {
         else
           *Brackets = *BI.Incoming;
       } else {
-        if (!Brackets)
+        if (!Brackets) {
           Brackets = std::make_unique<WaitcntBrackets>(
               ST, MaxCounter, Limits, WaitEventMaskForInst, SmemAccessCounter);
-        else
-          *Brackets = WaitcntBrackets(ST, MaxCounter, Limits,
-                                      WaitEventMaskForInst, SmemAccessCounter);
+        } else {
+          // Reinitialize in-place. N.B. do not do this by assigning from a
+          // temporary because the WaitcntBrackets class is large and it could
+          // cause this function to use an unreasonable amount of stack space.
+          Brackets->~WaitcntBrackets();
+          new (Brackets.get()) WaitcntBrackets(
+              ST, MaxCounter, Limits, WaitEventMaskForInst, SmemAccessCounter);
+        }
       }
 
       Modified |= insertWaitcntInBlock(MF, *MBB, *Brackets);