[llvm] [GlobalISel] Fix fewerElementsVectorPhi to insert after G_PHIs (PR #87927)

[Dávid Ferenc Szabó via llvm-commits]

https://github.com/dfszabo created https://github.com/llvm/llvm-project/pull/87927

Currently the inserted mergelike instructions will be inserted at the location of the G_PHI. Seems like the behaviour was correct before, but the rework done in https://reviews.llvm.org/D114198 forgot to include the part which makes sure the instructions will be inserted after all the G_PHIs.

>From 65ce2cff8bceb94a5e3a2e4c969b4a6bbb3dbfe2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?D=C3=A1vid=20Ferenc=20Szab=C3=B3?=
 <szabodavidferenc at gmail.com>
Date: Sun, 7 Apr 2024 17:59:17 +0200
Subject: [PATCH] [GlobalISel] Fix fewerElementsVectorPhi to insert after
 G_PHIs

Currently the inserted mergelike instructions will be inserted at the location of the G_PHI. Seems like the behaviour was correct before, but the rework done in https://reviews.llvm.org/D114198 forgot to include the part which makes sure the instructions will be inserted after all the G_PHIs.
---
 llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 95c6a359e52ec6..9412fa85cd3856 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -4265,6 +4265,10 @@ LegalizerHelper::fewerElementsVectorPhi(GenericMachineInstr &MI,
     }
   }
 
+  // Set the insert point after the existing PHIs
+  MachineBasicBlock &MBB = *MI.getParent();
+  MIRBuilder.setInsertPt(MBB, MBB.getFirstNonPHI());
+
   // Merge small outputs into MI's def.
   if (NumLeftovers) {
     mergeMixedSubvectors(MI.getReg(0), OutputRegs);