[llvm] [CodeGen][AMDGPU] Replace isBasicBlockPrologue with a MachineInstr::BBProlog flag (PR #225433)

[Matt Arsenault via llvm-commits]

================
@@ -210,13 +210,22 @@ MachineBasicBlock::iterator MachineBasicBlock::getFirstNonPHI() {
   return I;
 }
 
+void MachineBasicBlock::inheritBBProlog(iterator Begin, iterator End) {
----------------
arsenm wrote:

The shape and name of this function looks weird. I'd expect a test for isInProlog or something 

https://github.com/llvm/llvm-project/pull/225433