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

include/llvm/CodeGen/MachineRegisterInfo.h

@@ -273,6 +273,9 @@ public:
   ///
   unsigned getNumVirtRegs() const { return VRegInfo.size(); }
 
+  /// clearVirtRegs - Remove all virtual registers (after physreg assignment).
+  void clearVirtRegs();
+
   /// setRegAllocationHint - Specify a register allocation hint for the
   /// specified virtual register.
   void setRegAllocationHint(unsigned Reg, unsigned Type, unsigned PrefReg) {