Soften assertion in AMDGPU emitPrologue.

[niravd]

Summary

[AMDGPU] emitPrologue looks for an unused unallocated SGPR that is not
the scratch descriptor. Continue search if unused register found fails
other requirements.

This resolves the final test failures holding up the Consecutive Store
Merge cleanup (D14834).

[arsenm]

LGTM

lib/Target/AMDGPU/SIFrameLowering.cpp

if (ScratchWaveOffsetReg == TRI->reservedPrivateSegmentWaveByteOffsetReg(MF)) {
       //     register is being used for the scratch wave offset and there
       //     are no other free SGPRs, then the value will stay in this register.
       // ----
       //  13
       for (MCPhysReg Reg : getAllSGPRs().drop_back(13).slice(NumPreloaded)) {
         // Pick the first unallocated SGPR. Be careful not to pick an alias of the
         // scratch descriptor, since we haven’t added its uses yet.
         if (!MRI.isPhysRegUsed(Reg)) {
-          assert(MRI.isAllocatable(Reg) &&
-                !TRI->isSubRegisterEq(ScratchRsrcReg, Reg));
+          if (!MRI.isAllocatable(Reg) ||
+              TRI->isSubRegisterEq(ScratchRsrcReg, Reg))
+            continue;
 
           MRI.replaceRegWith(ScratchWaveOffsetReg, Reg);
           ScratchWaveOffsetReg = Reg;
           MFI->setScratchWaveOffsetReg(ScratchWaveOffsetReg);
           break;
         }
       }
     }