[MachineSink][RISCV] Only call isConstantPhysReg or isIgnorableUse for uses. (#99363)

The included test case contains X0 as a def register. X0 is considered a
constant register when it is a use. When its a def, it means to throw
away the result value.

If we treat it as a constant register here, we will execute the continue
and not assign `DefReg` to any register. This will cause a crash when
trying to get the register class for `DefReg` after the loop.

By only checking isConstantPhysReg for uses, we will reach the `return
false` a little further down and stop processing this instruction.

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index d782c8b..4b3ff57 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -417,7 +417,7 @@ bool MachineSinking::PerformSinkAndFold(MachineInstr &MI,
       continue;
     }
 
-    if (Reg.isPhysical() &&
+    if (Reg.isPhysical() && MO.isUse() &&
         (MRI->isConstantPhysReg(Reg) || TII->isIgnorableUse(MO)))
       continue;