[AMDGPU] Simplify Waitcnt constructor. NFC. (llvm#125672)

These fields are already initialized in their declarations.

Author: jayfoad

llvm/lib/Target/AMDGPU/Utils/AMDGPUBaseInfo.h

@@ -976,8 +976,7 @@ struct Waitcnt {
   Waitcnt() = default;
   // Pre-gfx12 constructor.
   Waitcnt(unsigned VmCnt, unsigned ExpCnt, unsigned LgkmCnt, unsigned VsCnt)
-      : LoadCnt(VmCnt), ExpCnt(ExpCnt), DsCnt(LgkmCnt), StoreCnt(VsCnt),
-        SampleCnt(~0u), BvhCnt(~0u), KmCnt(~0u) {}
+      : LoadCnt(VmCnt), ExpCnt(ExpCnt), DsCnt(LgkmCnt), StoreCnt(VsCnt) {}
 
   // gfx12+ constructor.
   Waitcnt(unsigned LoadCnt, unsigned ExpCnt, unsigned DsCnt, unsigned StoreCnt,