[RISCV][GISel] Reverse the operands the buildStore created in legalizeVAStart. (llvm#73989)

We need to store the frame index to the location pointed to by the
VASTART, not the other way around.

Author: topperc

llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp

@@ -406,7 +406,7 @@ bool RISCVLegalizerInfo::legalizeVAStart(MachineInstr &MI,
   LLT AddrTy = MIRBuilder.getMRI()->getType(MI.getOperand(0).getReg());
   auto FINAddr = MIRBuilder.buildFrameIndex(AddrTy, FI);
   assert(MI.hasOneMemOperand());
-  MIRBuilder.buildStore(MI.getOperand(0).getReg(), FINAddr,
+  MIRBuilder.buildStore(FINAddr, MI.getOperand(0).getReg(),
                         *MI.memoperands()[0]);
   MI.eraseFromParent();
   return true;

llvm/test/CodeGen/RISCV/GlobalISel/legalizer/legalize-vastart-rv32.mir

@@ -0,0 +1,29 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=riscv32 -run-pass=legalizer %s -o - | FileCheck --check-prefix=RV32 %s
+
+---
+name:            test_va_start
+tracksRegLiveness: true
+fixedStack:
+  - { id: 0, size: 4, alignment: 16, isImmutable: true }
+stack:
+  - { id: 0, size: 4, alignment: 4 }
+machineFunctionInfo:
+  varArgsFrameIndex: -1
+  varArgsSaveSize: 0
+body:             |
+  bb.1:
+    liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
+
+    ; RV32-LABEL: name: test_va_start
+    ; RV32: liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
+    ; RV32-NEXT: {{  $}}
+    ; RV32-NEXT: [[FRAME_INDEX:%[0-9]+]]:_(p0) = G_FRAME_INDEX %stack.0
+    ; RV32-NEXT: [[FRAME_INDEX1:%[0-9]+]]:_(p0) = G_FRAME_INDEX %fixed-stack.0
+    ; RV32-NEXT: G_STORE [[FRAME_INDEX]](p0), [[FRAME_INDEX1]](p0) :: (store (s32))
+    ; RV32-NEXT: PseudoRET
+    %8:_(p0) = G_FRAME_INDEX %stack.0
+    G_VASTART %8(p0) :: (store (s32))
+    PseudoRET
+
+...

llvm/test/CodeGen/RISCV/GlobalISel/legalizer/legalize-vastart-rv64.mir

@@ -0,0 +1,29 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=riscv64 -run-pass=legalizer %s -o - | FileCheck --check-prefix=RV64 %s
+
+---
+name:            test_va_start
+tracksRegLiveness: true
+fixedStack:
+  - { id: 0, size: 8, alignment: 16, isImmutable: true }
+stack:
+  - { id: 0, size: 8, alignment: 8 }
+machineFunctionInfo:
+  varArgsFrameIndex: -1
+  varArgsSaveSize: 0
+body:             |
+  bb.1:
+    liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
+
+    ; RV64-LABEL: name: test_va_start
+    ; RV64: liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
+    ; RV64-NEXT: {{  $}}
+    ; RV64-NEXT: [[FRAME_INDEX:%[0-9]+]]:_(p0) = G_FRAME_INDEX %stack.0
+    ; RV64-NEXT: [[FRAME_INDEX1:%[0-9]+]]:_(p0) = G_FRAME_INDEX %fixed-stack.0
+    ; RV64-NEXT: G_STORE [[FRAME_INDEX]](p0), [[FRAME_INDEX1]](p0) :: (store (s64))
+    ; RV64-NEXT: PseudoRET
+    %8:_(p0) = G_FRAME_INDEX %stack.0
+    G_VASTART %8(p0) :: (store (s64))
+    PseudoRET
+
+...