[Hexagon] Check if register is non-null before calling subregs_inclusive

[barannikov88]

Summary

D151036 adds an assertions that prohibits iterating over sub- and
super-registers of a null register. This is already the case when
iterating over register units of a null register, and worked by
accident for sub- and super-registers.

[barannikov88]

This is a workaround to fix these failures (caused D151036 to be reverted):
https://lab.llvm.org/buildbot/#/builders/38/builds/12037

This patch preserves the behavior, NFCI.
It should probably skip debug instructions instead, but I don't know how to prepare test for that.

[barannikov88]

Kindly ping

[barannikov88]

ping

[kparzysz]

Sorry, I missed this. LGTM, thanks!

llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp

for (const MachineInstr &MI : MBB) {
         // frame will be required, all frame indexes should be within alloc-
         // frame/deallocframe. Otherwise, the code that translates a frame
         // index into an offset would have to be aware of the placement of
         // the frame creation/destruction instructions.
         if (MO.isFI())
           return true;
         if (MO.isReg()) {
           Register R = MO.getReg();
+          // Debug instructions may refer to $noreg.
+          if (!R)
+            continue;
           // Virtual registers will need scavenging, which then may require
           // a stack slot.
           if (R.isVirtual())
             return true;
           for (MCPhysReg S : HRI.subregs_inclusive(R))
             if (CSR[S])
               return true;
           continue;