[AArch64] Fix the crash for PR79756

this crash is exposes when there is MOVKWi instruction in the head of a block,
but without MOVZWi

llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp

@@ -2213,6 +2213,9 @@ bool AArch64LoadStoreOpt::isMatchingMovConstInsn(MachineInstr &MemMI,
     // movz + movk hold a large offset of a Ld/St instruction.
     MachineBasicBlock::iterator B = MI.getParent()->begin();
     MachineBasicBlock::iterator MBBI = &MI;
+    // Skip the scene when the MI is the first instruction of a block.
+    if (MBBI == B)
+      return false;
     MBBI = prev_nodbg(MBBI, B);
     MachineInstr &MovzMI = *MBBI;
     if (MovzMI.getOpcode() == AArch64::MOVZWi) {

llvm/test/CodeGen/AArch64/large-offset-ldr-merge.mir

@@ -45,3 +45,23 @@ body:             |
     renamable $w0 = LDRBBroX killed renamable $x0, renamable $x8, 0, 0
     RET undef $lr, implicit $w0
 ...
+
+# Negative test: No MOVZWi used for the const offset
+---
+name:            LdOffset_missing_MOVZ
+tracksRegLiveness: true
+liveins:
+  - { reg: '$x0', virtual-reg: '' }
+body:             |
+  bb.0.entry:
+    liveins: $x0
+
+    ; CHECK-LABEL: name: LdOffset_missing_MOVZ
+    ; CHECK: liveins: $x0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: renamable $w8 = MOVKWi $w8, 15, 16, implicit-def $x8
+    ; CHECK-NEXT: renamable $w0 = LDRBBroX killed renamable $x0, killed renamable $x8, 0, 0
+    ; CHECK-NEXT: RET undef $lr, implicit $w0
+    renamable $w8 = MOVKWi $w8, 15, 16, implicit-def $x8
+    renamable $w0 = LDRBBroX killed renamable $x0, killed renamable $x8, 0, 0
+    RET undef $lr, implicit $w0