[SystemZ] Add check for INIT_UNDEF in getInstSizeInBytes (#134661)

Due to some optimization changes, INIT_UNDEF is making its way to
`getInstSizeInBytes` in `llvm/lib/Target/SystemZ/SystemZLongBranch.cpp`
but we do not have an exception there in the assert. Since INIT_UNDEF is
described as being similar to IMPLICIT_DEF and there is a check for
IMPLICIT_DEF, it seems logical to also add a check for INIT_UNDEF.

---------

Co-authored-by: Tony Tao <[email protected]>

Author: tltao

llvm/lib/Target/SystemZ/SystemZLongBranch.cpp

@@ -216,6 +216,7 @@ static unsigned getInstSizeInBytes(const MachineInstr &MI,
           // These do not have a size:
           MI.isDebugOrPseudoInstr() || MI.isPosition() || MI.isKill() ||
           MI.isImplicitDef() || MI.getOpcode() == TargetOpcode::MEMBARRIER ||
+          MI.getOpcode() == TargetOpcode::INIT_UNDEF ||
           // These have a size that may be zero:
           MI.isInlineAsm() || MI.getOpcode() == SystemZ::STACKMAP ||
           MI.getOpcode() == SystemZ::PATCHPOINT ||

llvm/test/CodeGen/SystemZ/init-undef.mir

@@ -0,0 +1,17 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# REQUIRES: asserts
+# RUN: llc -run-pass systemz-long-branch -mtriple=s390x-ibm-zos -o - %s | FileCheck %s
+
+# Makes sure INIT_UNDEF does not crash the compiler in systemz-long-branch
+---
+name: func0
+body: |
+  bb.0.entry:
+    ; CHECK-LABEL: name: func0
+    ; CHECK: renamable $r0l = INIT_UNDEF
+    ; CHECK-NEXT: renamable $r3d = LGFR killed renamable $r0l
+    ; CHECK-NEXT: frame-destroy Return_XPLINK implicit $r3d
+    renamable $r0l = INIT_UNDEF
+    renamable $r3d = LGFR killed renamable $r0l
+    frame-destroy Return_XPLINK implicit $r3d
+...