Clear virtual registers after they are no longer referenced.

Passes after RegAlloc should be able to rely on MRI->getNumVirtRegs() == 0.
This makes sharing code for pre/postRA passes more robust.
Now, to check if a pass is running before the RA pipeline begins, use MRI->isSSA().
To check if a pass is running after the RA pipeline ends, use !MRI->getNumVirtRegs().

PEI resets virtual regs when it's done scavenging.

PTX will either have to provide its own PEI pass or assign physregs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151032 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/PTX/PTXTargetMachine.cpp b/lib/Target/PTX/PTXTargetMachine.cpp
index 61285b1..9305377 100644
--- a/lib/Target/PTX/PTXTargetMachine.cpp
+++ b/lib/Target/PTX/PTXTargetMachine.cpp
@@ -126,7 +126,9 @@
 } // namespace
 
 TargetPassConfig *PTXTargetMachine::createPassConfig(PassManagerBase &PM) {
-  return new PTXPassConfig(this, PM);
+  PTXPassConfig *PassConfig = new PTXPassConfig(this, PM);
+  PassConfig->disablePass(PrologEpilogCodeInserterID);
+  return PassConfig;
 }
 
 bool PTXPassConfig::addInstSelector() {