[ARM] Test unroll behaviour on machines with low overhead branching (#118692)

Add test for existing loop unroll behaviour.

Current behaviour is the single loop with fmul gets runtime unrolled by
count of 4, with the loop remainder unrolled as the 3 for.body9.us.prol
sections. This is quite a lot of compare and branch, negating the
benefits of the low overhead loop mechanism.

Author: VladiKrapp-Arm

llvm/test/Transforms/LoopUnroll/ARM/lob-unroll.ll

@@ -0,0 +1,63 @@
+; RUN: opt -mcpu=cortex-m55 -mtriple=thumbv8.1m.main -passes=loop-unroll -S  %s -o - | FileCheck %s --check-prefix=LOB
+
+; This test checks behaviour of loop unrolling on processors with low overhead branching available 
+
+; LOB-CHECK-LABEL: for.body{{.*}}.prol
+; LOB-COUNT-1:     fmul fast float 
+; LOB-CHECK-LABEL: for.body{{.*}}.prol.1
+; LOB-COUNT-1:     fmul fast float 
+; LOB-CHECK-LABEL: for.body{{.*}}.prol.2
+; LOB-COUNT-1:     fmul fast float 
+; LOB-CHECK-LABEL: for.body{{.*}}
+; LOB-COUNT-4:     fmul fast float 
+; LOB-NOT:     fmul fast float 
+
+; Function Attrs: nofree norecurse nosync nounwind memory(argmem: readwrite)
+define dso_local void @test(i32 noundef %n, ptr nocapture noundef %pA) local_unnamed_addr #0 {
+entry:
+  %cmp46 = icmp sgt i32 %n, 0
+  br i1 %cmp46, label %for.body, label %for.cond.cleanup
+
+for.cond.loopexit:                                ; preds = %for.cond6.for.cond.cleanup8_crit_edge.us, %for.body
+  %exitcond49.not = icmp eq i32 %add, %n
+  br i1 %exitcond49.not, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.cond.loopexit, %entry
+  ret void
+
+for.body:                                         ; preds = %entry, %for.cond.loopexit
+  %k.047 = phi i32 [ %add, %for.cond.loopexit ], [ 0, %entry ]
+  %add = add nuw nsw i32 %k.047, 1
+  %cmp244 = icmp slt i32 %add, %n
+  br i1 %cmp244, label %for.cond6.preheader.lr.ph, label %for.cond.loopexit
+
+for.cond6.preheader.lr.ph:                        ; preds = %for.body
+  %invariant.gep = getelementptr float, ptr %pA, i32 %k.047
+  br label %for.cond6.preheader.us
+
+for.cond6.preheader.us:                           ; preds = %for.cond6.for.cond.cleanup8_crit_edge.us, %for.cond6.preheader.lr.ph
+  %w.045.us = phi i32 [ %add, %for.cond6.preheader.lr.ph ], [ %inc19.us, %for.cond6.for.cond.cleanup8_crit_edge.us ]
+  %mul.us = mul nuw nsw i32 %w.045.us, %n
+  %0 = getelementptr float, ptr %pA, i32 %mul.us
+  %arrayidx.us = getelementptr float, ptr %0, i32 %k.047
+  br label %for.body9.us
+
+for.body9.us:                                     ; preds = %for.cond6.preheader.us, %for.body9.us
+  %x.043.us = phi i32 [ %add, %for.cond6.preheader.us ], [ %inc.us, %for.body9.us ]
+  %1 = load float, ptr %arrayidx.us, align 4
+  %mul11.us = mul nuw nsw i32 %x.043.us, %n
+  %gep.us = getelementptr float, ptr %invariant.gep, i32 %mul11.us
+  %2 = load float, ptr %gep.us, align 4
+  %mul14.us = fmul fast float %2, %1
+  %arrayidx17.us = getelementptr float, ptr %0, i32 %x.043.us
+  store float %mul14.us, ptr %arrayidx17.us, align 4
+  %inc.us = add nuw nsw i32 %x.043.us, 1
+  %exitcond.not = icmp eq i32 %inc.us, %n
+  br i1 %exitcond.not, label %for.cond6.for.cond.cleanup8_crit_edge.us, label %for.body9.us
+
+for.cond6.for.cond.cleanup8_crit_edge.us:         ; preds = %for.body9.us
+  %inc19.us = add nuw nsw i32 %w.045.us, 1
+  %exitcond48.not = icmp eq i32 %inc19.us, %n
+  br i1 %exitcond48.not, label %for.cond.loopexit, label %for.cond6.preheader.us
+}
+